[VS2010] C# 4.0 语言新功能：Named Argument and Optional Parameters（vs写c#语言）

25-01-30 25

本文的目的是介绍[VS2010]C#4.0语言新功能：NamedArgumentandOptionalParameters的详细情况，特别关注vs写c#语言的相关信息。我们将通过专业的研究、有关数据的

本文的目的是介绍[VS2010] C# 4.0 语言新功能：Named Argument and Optional Parameters的详细情况，特别关注vs写c#语言的相关信息。我们将通过专业的研究、有关数据的分析等多种方式，为您呈现一个全面的了解[VS2010] C# 4.0 语言新功能：Named Argument and Optional Parameters的机会，同时也不会遗漏关于"Parameter" vs "Argument"、@RequestParam 注解使用：Name for argument type [java.lang.String] not available, and parameter name inf...、Angular 4/2 Http get() Parameters + Headers + URLSearchParams + RequestOptions Exam、Angular 依赖注入领域里 optional constructor parameters 的概念介绍的知识。

本文目录一览：

[VS2010] C# 4.0 语言新功能：Named Argument and Optional Parameters（vs写c#语言）
"Parameter" vs "Argument"
@RequestParam 注解使用：Name for argument type [java.lang.String] not available, and parameter name inf...
Angular 4/2 Http get() Parameters + Headers + URLSearchParams + RequestOptions Exam
Angular 依赖注入领域里 optional constructor parameters 的概念介绍

[VS2010] C# 4.0 语言新功能：Named Argument and Optional Parameters（vs写c#语言）

[VS2010] C# 4.0 语言新功能：Named Argument and Optional Parameters

如果你是曾经用过 C# 来开发 Office 应用程序的开发人员，想必对这种程序一定很熟悉：

this.SaveAs(@"C:Book1.xml",Excel.XlFileFormat.xlXMLSpreadsheet,
??? Type.Missing,Type.Missing,Excel.XlSaveAsAccessMode.xlNoChange,
??? Type.Missing,Type.Missing);

这是因为到了 C# 3.0 为止，它都不支持选择性参数 (Optional Parameters) 的功能，因此在 Office 应用程序开发时，所有的选择性参数都一定要使用 Type.Missing 作为参数代表不设定，而 Office 对象模型中到处都是这种有一大堆选择性参数的函数与方法，最根本的原因是对象模型本身是为了 VBA (Visual Basic for Applications) 语言来设计的，VB/VBA 都支持选择性参数，所以相同的程序在 VB 中只要这样：

Me.SaveAs(FileName:="C:Book1.xml",FileFormat:=Excel.XlFileFormat.xlXMLSpreadsheet,AccessMode:=Excel.XlSaveAsAccessMode.xlNoChange)

在上面的程序中我们也看到了另一个只有 VB/VBA 才有的功能：具名参数 (Named Argument)，它可以在不按照参数顺序的情况下，直接以参数名称来设定参数的值，所以用 VB/VBA 开发 Office 应用程序比 C# 要简单非常非常多，对 C# 的开发人员其实也不太公平。

不过，这样不公平的情况，到了 C# 4.0 (VS 2010) 时，将会有很大的变化，因为 C# 4.0 开始支持了 Named Argument 以及 Optional Parameters 两种功能，以上面的那行程序来看，用 C# 4.0 可以写成这样：

this.SaveAs(FileName: @"C:Book1.xml",FileFormat: Excel.XlFileFormat.xlXMLSpreadsheet,AccessMode: Excel.XlSaveAsAccessMode.xlNoChange);

如何？是不是和 VB 一样简单了呢？

具名参数（Named Argument）

具名参数顾名思义，就是直接使用参数的名称来设定参数值，这个功能在 VB/VBA 中已经是司空见惯，并且被 C# 开发人员视为相当称羡的功能之一，C# 4.0 编译器以及 Visual Studio 2010 编辑界面开始支持具名参数，例如下列的函数：

public int MyFunction(int ArgA,int ArgB,int ArgC)

{

??? return ArgA * ArgB * ArgC;

}

开发人员可以用下列三种方式调用：

MyFunction(ArgA: 5,ArgB: 12,ArgC: 24); // return 5 * 12 * 24

MyFunction(ArgC: 5,ArgA: 12,ArgB: 24); // return 12 * 24 * 5

MyFunction(ArgB: 5,ArgC: 12,ArgA: 24); // return 24 * 5 * 12

选择性参数（Optional Parameters）

选择性参数也是 VB/VBA，以及 C/C++ 有的功能，只是 C# 一直都没有加进来，新的 C# 4.0 开始加入了选择性参数的功能，例如下列的函数：

public int MyFunction(int ArgA,int ArgB = 12,int ArgC = 5)

{

??? return ArgA * ArgB * ArgC;

}

下列调用方式会有不同的值：

MyFunction(14,22,43); // return 14 * 22 * 43

MyFunction(16); // return 16 * 12 * 5

MyFunction(ArgB: 25); // 错误，因为 ArgA 没有指定参数。

MyFunction(16,ArgC: 100); // return 16 * 12 * 100

选择性参数最直接的好处，就是不用再写一大堆的 Type.Missing，就可以轻松的调用 COM 的对象方法了。例如下列的函数：

var excelApp = new Microsoft.Office.Interop.Excel.Application();

var myFormat = Microsoft.Office.Interop.Excel.XlRangeAutoFormat.xlRangeAutoFormatAccounting1;

excelApp.get_Range("A1","B4").AutoFormat(myFormat,Type.Missing);

到了 C# 4.0，只要这样写就好了：

var excelApp = new Microsoft.Office.Interop.Excel.Application();

var myFormat = Microsoft.Office.Interop.Excel.XlRangeAutoFormat.xlRangeAutoFormatAccounting1;

excelApp.get_Range("A1","B4").AutoFormat( Format: myFormat );

不过请务必记住一件事，Named Argument 和 Optional Parameters 虽然好用，但绝对不要滥用，否则会对程序可读性有相当大的伤害。

原文:大专栏 [VS2010] C# 4.0 语言新功能：Named Argument and Optional Parameters

"Parameter" vs "Argument"

我把 “Parameter” vs “Argument”弄混了，并没有真正注意什么时候使用一个，什么时候使用另一个。

你能告诉我吗？

@RequestParam 注解使用：Name for argument type [java.lang.String] not available, and parameter name inf...

详细错误信息

Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.IllegalArgumentException: Name for argument type [java.lang.String] not available, and parameter name information not found in class file either.] with root cause java.lang.IllegalArgumentException: Name for argument type [java.lang.String] not available, and parameter name information not found in class file either.

2018-03-01 03:39:35.038 ERROR org.apache.catalina.core.ContainerBase.[Tomcat].[localhost].[/].[dispatcherServlet][http-nio-8080-exec-1]: Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.IllegalArgumentException: Name for argument type [java.lang.String] not available, and parameter name information not found in class file either.] with root cause java.lang.IllegalArgumentException: Name for argument type [java.lang.String] not available, and parameter name information not found in class file either.
	at org.springframework.web.method.annotation.AbstractNamedValueMethodArgumentResolver.updateNamedValueInfo(AbstractNamedValueMethodArgumentResolver.java:168)
	at org.springframework.web.method.annotation.AbstractNamedValueMethodArgumentResolver.getNamedValueInfo(AbstractNamedValueMethodArgumentResolver.java:145)
	at org.springframework.web.method.annotation.AbstractNamedValueMethodArgumentResolver.resolveArgument(AbstractNamedValueMethodArgumentResolver.java:94)
	at org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:121)
	at org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:158)
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:128)
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:97)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:967)
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901)
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.springframework.web.filter.CorsFilter.doFilterInternal(CorsFilter.java:96)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:478)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459)
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
	at java.lang.Thread.run(Thread.java:748)

解决思路

需要指定 @RequestParam 的 name 属性值

源码分析

AbstractNamedValueMethodArgumentResolver.java

private NamedValueInfo getNamedValueInfo(MethodParameter parameter) {
    NamedValueInfo namedValueInfo = this.namedValueInfoCache.get(parameter);
    if (namedValueInfo == null) {
        namedValueInfo = createNamedValueInfo(parameter);
        namedValueInfo = updateNamedValueInfo(parameter, namedValueInfo);
        this.namedValueInfoCache.put(parameter, namedValueInfo);
    }
    return namedValueInfo;
}

/**
 * Create the {@link NamedValueInfo} object for the given method parameter. Implementations typically
 * retrieve the method annotation by means of {@link MethodParameter#getParameterAnnotation(Class)}.
 * @param parameter the method parameter
 * @return the named value information
 */
protected abstract NamedValueInfo createNamedValueInfo(MethodParameter parameter);

/**
 * Create a new NamedValueInfo based on the given NamedValueInfo with sanitized values.
 */
private NamedValueInfo updateNamedValueInfo(MethodParameter parameter, NamedValueInfo info) {
    String name = info.name;
    if (info.name.isEmpty()) {
        name = parameter.getParameterName();
        if (name == null) {
            throw new IllegalArgumentException(
                    "Name for argument type [" + parameter.getNestedParameterType().getName() +
                    "] not available, and parameter name information not found in class file either.");
        }
    }
    String defaultValue = (ValueConstants.DEFAULT_NONE.equals(info.defaultValue) ? null : info.defaultValue);
    return new NamedValueInfo(name, info.required, defaultValue);
}


RequestParamMethodArgumentResolver.java

@Override
protected NamedValueInfo createNamedValueInfo(MethodParameter parameter) {
    RequestParam ann = parameter.getParameterAnnotation(RequestParam.class);
    return (ann != null ? new RequestParamNamedValueInfo(ann) : new RequestParamNamedValueInfo());
}


private static class RequestParamNamedValueInfo extends NamedValueInfo {

    public RequestParamNamedValueInfo() {
        super("", false, ValueConstants.DEFAULT_NONE);
    }

    public RequestParamNamedValueInfo(RequestParam annotation) {
        super(annotation.name(), annotation.required(), annotation.defaultValue());
    }
}

Angular 4/2 Http get() Parameters + Headers + URLSearchParams + RequestOptions Exam

https://www.concretepage.com/angular-2/angular-2-http-get-parameters-headers-urlsearchparams-requestoptions-example

By Arvind Rai, May 19,2017

This page will walk through Angular 2 Http get() parameters + Headers + URLSearchParams + RequestOptions example. Angular Headers class is used to create headers. Angular URLSearchParams class is used to create URL parameters. Angular RequestOptions instantiates itself using instances of

, 
and other request options such as url,method,search,body,withCredentials,responseType. These classes are imported from 
@angular/http 
API. Finally 
Http.get() 
uses instance of 
to interact with the server. Though 
is optional to use with 
arameters in the URL,we need to use them. On this page we will create an application that will use 
to send headers and parameters using angular in-memory web API. Find the code snippet from our example. 
 
  
  
  
  
 getBookById(bookId: string): Observable<Book[]> {
    let myHeaders = new Headers();
    myHeaders.append('Content-Type', 'application/json');    
    let myParams URLSearchParams();
    myParams'id' bookId);	
    let options RequestOptions({ headers: myHeaders params: myParams });
    return this.http.get(this.url options)
	.map.extractData.catch.handleError);
}  
 
Using 
set() 
or 
append() 
method of 
and 
arameters and headers,too. Now we will walk through complete example step by step. 
 
  Contents 
  
   Technologies Used 
   Headers 
   URLSearchParams 
   RequestOptionsArgs and RequestOptions 
   Http.get() with Multiple Headers and Multiple Parameters 
   Angular In-Memory Web API 
   Complete Example 
   Run Application 
   References 
   Download Source Code 
  
 

 
 
 
 Technologies Used 
Find the technologies being used in our example. 

 
1. Angular 4.0.0 

2. TypeScript 2.2.0 

3. Node.js 6.10.1 

4. Angular CLI 1.0.0 

5. Angular Compiler CLI 4.0.0 

6. angular-in-memory-web-api@0.3.2 
 Headers 
is the angular class that is used to configure request headers. Find the sample 
instantiation. 
let myHeaders ();  
 
We can also pass headers as an argument while instantiating 
class. Find the code snippet. 
({ : 'Cache-Control''no-cache' });  
 
To fetch,add and delete headers,sans-serif; font-size:14px">class has following methods. 

append(name: string,value: string) 
: Appends a header to existing list of header values for a given header name. We use 
as follows. 
myHeaders'Accept''text/plain');    
myHeaders' application/xhtml+xml ');  
 
Now the 
Accept 
header will have the following values. 
Accept: text/plain application/xhtml+xml  
 
set(name: string,value: string|string[]) 
: Sets or overrides header value for given name. It is used as follows. 
.set' application/xml 'header will have only the following value. 
: application/xml  
 
delete(name: string) 
: Deletes all header values for the given name. We use it as follows. 
.deleteget(name: string) : string 
: Returns first header that matches given name. Find the code snippet. 
let acceptHeader =  myHeadersgetAll(name: string) : string[] 
: Returns list of header values for a given name. 
let acceptHeaders .getAll );  
 

If we want to add multiple headers,we can achieve it by 
method as follows. 
);
myHeaders); 
 
If we want to add multiple headers by 
method,we can achieve it as follows. 
); 
 

 
 
 
 URLSearchParams 
creates the query string in the URL. It is a map-like representation of URL search parameters. Find its constructor Syntax. 
constructor(rawParams?: string queryEncoder?: QueryEncoder)  
 
Both arguments in the constructor are optional. Angular 
queryEncoder 
parameter is used to pass any custom 
QueryEncoder 
to encode key and value of the query string. By default 
encodes keys and values of parameter using JavaScript 
encodeURIComponent() 
method. 

Now we can instantiate 
as given below. 
let myParams Now we can fetch,add and delete parameters using following methods. 

append(param: string,val: string) : void 
: Appends parameter value to existing list of parameter values for a given parameter name. It is used to add values in multi-value fields or arrays in query string. If we write the code as given below. 
myParams'names''John');
myParams'David'Then query parameter 
names 
will be an array. The query string will look like as given below. 
?names[]=John&namesDavid  
 
Server side code such as PHP will get 
parameter value as an array. 

set(param: string,val: string) 
: Sets or overrides parameter value for given parameter name. We can use as follows. 
'Bob'The query string will be as follows. 
=Bob  
 
delete(param: string) : void 
: Deletes all parameter values for the given parameter name. Find the code snippet. 
get(param: string) : string 
: In case of multi-value fields,it returns the first value for given parameter name. Find the code snippet. 
let nameParam = myParamsgetAll(param: string) : string[] 
: Returns list of values for a given parameter name. Find the code snippet. 
let namesParam .getAllIf we want to add multiple parameters,0)">'category' catg'writer' wtrIf we want to add multiple parameters by 
); 
 
 RequestOptionsArgs and RequestOptions 
RequestOptionsArgs 
is an interface that is used to construct a 
. The fields of 
are url,params,headers,responseType. 

 
is used to create request option. It is instantiated using 
. It contains all the fields of the 
interface. Now find the constructor of 
class. 
({method headers body url search 
                 withCredentials responseType}?: RequestOptionsArgsIn our example we will use following fields. 

headers 
: Sets headers for HTTP request. It is of 
class type. 

params 
: Sets query parameters in the URL. It is of 
 

Now if we have instance of 
();
myHeadersAnd instance of 
();
myParamsThen 
headers 
params 
can be passed to 
let options });  
 
 Http.get() with Multiple Headers and Multiple Parameters 
Angular 
method performs a request with HTTP GET method. Find the arguments of 
method. 
get(url) : Response>  
 
url 
: This is the HTTP URL to hit the server using HTTP GET method. 

RequestOptionsArgs 
: This is optional in 
method. This is used to create instance of 
to send headers,parameters etc with 
Now If we want to add multiple headers,we can do as follows. 
(); 
myHeaders);	  
 
Find the code snippet for 
with multiple headers and multiple URL parameters. 
getBooksAfterFilter(catg{
  let myHeaders (); 
  myHeaders);
  myHeaders);  
  let myParams ();
  myParams);
  myParams);		
  let options });
  return this} 
 
 Angular In-Memory Web API 
Angular provides in-memory web API to process HTTP request in test environment. In case we don't have actual server URL,we can use angular in-memory web API for testing our angular 
Http 
methods. It provides a dummy URL which can be changed by actual URL later. It returns an 
Observable 
of HTTP 
Response 
object in the manner of a RESTy web api. In our example we are using in-memory web API to get and post data. To use it in our angular application we need to follow below steps. 

Step-1 
: Add 
angular-in-memory-web-api 
in 
dependencies 
block in 
package.json 
file as given below. 
"angular-in-memory-web-api""~0.3.2"  
 
Step-2 
: Run 
npm install 
command to download 
. 

Step-3 
: Create a class implementing 
InMemoryDbService 
interface. In our example we are creating an in-memory DB for books. Find our class for our in-memory DB. 

book-data.ts 
 
import { InMemoryDbService } from 'angular-in-memory-web-api';

export class BookData implements {
  createDb() {
    let books = [
      { id'1' name'Angular 2 by Krishna' category'Angular' writer'Krishna' },
      '2''AngularJS by Krishna''3''Angular 2 by Vishnu''Vishnu' 	
      '4''Core Java by Vishnu''Java''5''JSP & Servlet by Vishnu''6''JPA by Vishnu''7''Hibernate by Krishna''Hibernate'}
    ];
    return {books};
  }
To interact with DB,URL will be 
api/books 
Step-4 
: Before using DB we need to configure our above class in application module using 
imports 
Metadata of 
@NgModule 
InMemoryWebApiModule.forRoot(BookDataFind the application module. 
InMemoryWebApiModule ;
import BookData './book-data';

@NgModule({
---------
  imports: [     
        browserModule
	HttpModule)
  ]
---------
})  
 
Find the 
link 
for more information on in-memory web API. 

 
 

  
 
 Complete Example 
Find the complete example. 

book.service.ts 
Injectable '@angular/core'Http RequestOptions '@angular/http'Observable 'rxjs';

import Book './book'@Injectable()
export class BookService {
    url = "api/books";
    constructor(private http:{ }
    getAllBooks(): {
        return this)
		 );
    }
    getBookById{
	let myHeaders ();
	myHeaders);    
	let myParams ();
	myParams);			
        let options });
        return this)
	        }
    getBooksAfterFilter);   
	let myParams );
	myParams);		
        let options }
    private extractData(res{
	let body = res.json();
        return body.data;
    }
    private handleError (errorResponse | any{
	console.error.message || error);
	return .throwbook.component.ts 
ComponentOnInit NgForm '@angular/forms''./book.service'@Component({
   selector'app-book'
   templateUrl'./book.component.html'
   styleUrls['./book.component.css'})
export class BookComponent implements { 
   allBooks[];
   book;
   filteredlistofBooks[];
   errorMessageString;
   dataAvailableById= true;
   dataAvailableAfterFilter;
   categories [ 
                 {name
                 
		 }
		];
   writers [ 
               'Krishna'
               'Vishnu'}
             ];
   constructor(private bookServiceBookService}
   
   ngOnInit(): void {
        this.getAllBooks();
   }
   getAllBooks.bookService()
	  .subscribe(
                data => this.allBooks = data
                error =>  this.errorMessage <any>error);   
   }
   getBookById.dataAvailableById;
	this.book = null;
        this.getBookById)
	    => {  
        	    if(data.length > 0{
		        this[]; 
		    } else {
			this= false; 
		    }	
	        <any>error
	     );     
   }
   getBooksAfterFilter(category.dataAvailableAfterFilter.filteredlistofBooks .getBooksAfterFilter)
	  {  
		    if}	
		<any>error
	  );    
   }
   bookById(bookByIdFormNgForm{
	  let bookId = bookByIdForm.controls'bookId'].value;
	  this);
   }
   filterBooks{
         let catg ;
	 let wtr ;
	 thisbook.component.html 
<h3>Book Details</h3>
<table>
  <tr><th> Id</th> <th>Name</th><th>Category</th><th>Writer</th></tr>
  <tr *ngFor="let bk of allBooks" >
    <td>{{bk.id}}</td> <td>{{bk.name}}<td>{{bk.category}}<td>{{bk.writer}}</td>
  </tr>
</table>
<h3>Get Book by ID <div>
  <form #bookByIdForm= "ngForm" (ngSubmit)"bookById(bookByIdForm)">
	 <div> 
	  Enter Book Id: <input name"bookId" ngModel required #bookId"ngModel"</div> 
	 <div> <br/> 	  
	   <button [disabled]"bookByIdForm.invalid">Submit</button>
	 </div>	  
  </form>
</div>  
<br/>
<div *ngIf"bookByIdForm.submitted""book; else loading">
	<table>
	  </th></tr>
	  <tr>
		<td>{{book.id}}<td>{{book.name}}<td>{{book.category}}<td>{{book.writer}}</td>
	  </tr>
	</table>				
    </div>   
    <ng-template #loading"dataAvailableById; else notAvailable">
	       Loading data...
	</div>   
	notAvailable> Data not Aavailable. </ng-template>   
    </ng-template>   
</div>
<h3>Filter Books filterBookForm"filterBooks(filterBookForm)">
        <div> 
	  Category:
	  <select "category" ngModel>
             <option value"" disabled>Select a Category</option>
             <option *"let category of categories" [ngValue]"category.name">
                {{ category.name }}
             </option>
          </select>
	</div> <br/>
	<div> 
	  Writer:
	  "writer" >
            >Select a Writer</option>
            "let writer of writers" ["writer.name">
               {{ writer.name }}
            </div> 
	<div><br/> 	  
	   <button>Submit</button>
	"filterBookForm.submitted""filteredlistofBooks; else loading""let bk of filteredlistofBooks" >
		</table>		
    "dataAvailableAfterFilter; else notAvailable"</div>

"errorMessage" [ngClass] "'error'"> {{errorMessage}} </div>  
 
book.component.css 
table {
    border-collapse: collapse;
}
table th td : 1px solid black{
    color: red;
    font-size20pxbook.ts 
export class {
   id;
   name;
   category;
   writer;
   constructor{ 
   }   
 
app.component.ts 
Component 'app-root'
   template`
		<app-book></app-book>
             `
AppComponent { 

app.module.ts 
NgModule browserModule '@angular/platform-browser'FormsModule HttpModule }  from './app.component'BookComponent './book.component'({
  importsFormsModule],
  declarations[
        AppComponentBookComponent
  
  providersBookService
  
  bootstrapAppComponent
  AppModule }  
 
 Run Application 
To run the application,find following steps. 

1. 
Download source code using download link given on this page. 

2. 
In your angular CLI application,replace 
src 
folder. 

3. 
Add 
"angular-in-memory-web-api": "~0.3.2" 
file. 

4. 
Run 
and then run 
ng serve 
5. 
Now access the URL 
http://localhost:4200 
. Find the print screen. 
 
  
 

Find the link for Angular 2 
CRUD operation with Spring Boot. 

 
Spring Boot REST + Angular 2 + JPA + Hibernate + MySQL CRUD Example 
 
 References 
Http 
 

 
Headers 
 
URLSearchParams 
 
RequestOptions 
 
Angular 2 Http post() Example 
 
 Download Source Code 
 
 angular-2-http-get-parameters-headers-urlsearchparams-requestoptions-example.zip 

                  


                  总结
                  以上是小编为你收集整理的Angular 4/2 Http get() Parameters + Headers + URLSearchParams + RequestOptions Exam全部内容。
                  如果觉得小编网站内容还不错，欢迎将小编网站推荐给好友。

            

          

          

          
            
              

                
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Angular 依赖注入领域里 optional constructor parameters 的概念介绍

Angular 依赖注入（DI）是一个强大且灵活的设计模式，它可以帮助我们更好地管理和组织我们的代码。构造函数参数的可选性（Optional）是 Angular DI 系统的一个重要特性。这种特性允许我们将某些服务或值作为可选依赖注入到组件或服务中，这样，如果这些服务或值不存在，我们的代码仍然可以正常工作。

首先，让我们来理解一下 Angular 的依赖注入系统。在 Angular 中，我们通常通过构造函数来注入依赖。例如，如果我们有一个服务 UserService，我们可以在组件的构造函数中注入它：

constructor(private userService: UserService) { }

然后，Angular 的依赖注入系统会负责创建 UserService 的一个实例，并将其注入到我们的组件中。这样，我们就可以在组件中使用这个服务了。

然而，有时候我们可能想要将一个服务作为可选依赖注入。也就是说，如果这个服务存在，我们就使用它；如果不存在，我们的代码仍然可以正常运行。这就是 @Optional() 装饰器的作用。@Optional() 装饰器告诉 Angular 的依赖注入系统，这个依赖是可选的，如果找不到这个依赖，那么就注入 null。

下面是一个使用 @Optional() 的例子：

import { Optional } from ''@angular/core'';

constructor(@Optional() private userService: UserService) { }

在这个例子中，如果 UserService 没有提供，那么 userService 就会被设置为 null。然后，我们就需要在使用 userService 之前进行空检查，以防止运行时错误。

关于 Angular 的依赖注入系统，尤其是可选构造函数参数，还有许多其他的细节和高级特性，这些都超出了本文的范围。但是，我希望这个简单的介绍能帮助你理解 @Optional() 的基本概念，以及它在 Angular 应用中的用途。

Angular 的依赖注入系统提供了一种强大且灵活的方式来管理和组织我们的代码。通过理解和利用这些特性，我们可以编写出更清晰、更容易维护的代码。

我们今天的关于[VS2010] C# 4.0 语言新功能：Named Argument and Optional Parameters和vs写c#语言的分享就到这里，谢谢您的阅读，如果想了解更多关于"Parameter" vs "Argument"、@RequestParam 注解使用：Name for argument type [java.lang.String] not available, and parameter name inf...、Angular 4/2 Http get() Parameters + Headers + URLSearchParams + RequestOptions Exam、Angular 依赖注入领域里 optional constructor parameters 的概念介绍的相关信息，可以在本站进行搜索。

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