Type.Missing Feld

public: static initonly System::Object ^ Missing;

public static readonly object Missing;

 staticval mutable Missing : obj

Public Shared ReadOnly Missing As Object 

Feldwert

Beispiele

Das folgende Codebeispiel zeigt die Verwendung des Missing Felds zum Aufrufen einer Methode mit ihren Standardargumenten.

using System;
using System.Reflection;
using System.CodeDom.Compiler;

class Example
{
    public static void Main()
    {
        // VB source for example. Not all versions of CS and CPP compilers
        // support optional arguments.
        string codeLines =
            "Imports System\n\n" +
            "Public Class OptionalArg\n" +
            "  Public Sub MyMethod(ByVal a As Integer, _\n" +
            "    Optional ByVal b As Double = 1.2, _\n" +
            "    Optional ByVal c As Integer = 1)\n\n" +
            "    Console.WriteLine(\"a = \" & a & \" b = \" & b & \" c = \" & c)\n" +
            "  End Sub\n" +
            "End Class\n";

        // Generate a OptionalArg instance from the source above.
        object o = GenerateObjectFromSource("OptionalArg", codeLines, "VisualBasic");
        Type t;

        t = o.GetType();
        BindingFlags bf = BindingFlags.Public | BindingFlags.Instance |
            BindingFlags.InvokeMethod | BindingFlags.OptionalParamBinding;

        t.InvokeMember("MyMethod", bf, null, o, new object[] {10, 55.3, 12});
        t.InvokeMember("MyMethod", bf, null, o, new object[] {10, 1.3, Type.Missing});
        t.InvokeMember("MyMethod", bf, null, o, new object[] {10, Type.Missing, Type.Missing});
    }

    private static object GenerateObjectFromSource(string objectName,
        string sourceLines, string providerName)
    {
        object genObject = null;
        CodeDomProvider codeProvider = CodeDomProvider.CreateProvider(providerName);
        CompilerParameters cp = new CompilerParameters();

        cp.GenerateExecutable = false;
        cp.GenerateInMemory = true;

        CompilerResults results =
            codeProvider.CompileAssemblyFromSource(cp, sourceLines);
        if (results.Errors.Count == 0)
        {
            genObject = results.CompiledAssembly.CreateInstance(objectName);
        }

        return genObject;
    }
}

open System
open System.Reflection
open System.CodeDom.Compiler

let generateObjectFromSource objectName (sourceLines: string) providerName =
    let codeProvider = CodeDomProvider.CreateProvider providerName
    let cp = CompilerParameters()

    cp.GenerateExecutable <- false
    cp.GenerateInMemory <- true

    let results = codeProvider.CompileAssemblyFromSource(cp, sourceLines)
    if results.Errors.Count = 0 then
        results.CompiledAssembly.CreateInstance objectName
    else 
        null


// VB source for example. Not all versions of CS and CPP compilers
// support optional arguments.
let codeLines =
    """Imports System
Public Class OptionalArg
  Public Sub MyMethod(ByVal a As Integer, _
    Optional ByVal b As Double = 1.2, _
    Optional ByVal c As Integer = 1)

    Console.WriteLine(\"a = \" & a & \" b = \" & b & \" c = \" & c)
  End Sub
End Class"""

// Generate a OptionalArg instance from the source above.
let o = generateObjectFromSource "OptionalArg" codeLines "VisualBasic"

let t = o.GetType()
let bf = BindingFlags.Public ||| BindingFlags.Instance ||| BindingFlags.InvokeMethod ||| BindingFlags.OptionalParamBinding

t.InvokeMember("MyMethod", bf, null, o, [| 10; 55.3; 12 |]) |> ignore
t.InvokeMember("MyMethod", bf, null, o, [| 10; 1.3; Type.Missing |]) |> ignore
t.InvokeMember("MyMethod", bf, null, o, [| 10; Type.Missing; Type.Missing |]) |> ignore

Imports System.Reflection

Public Class OptionalArg
    Public Sub MyMethod(ByVal a As Integer, _
        Optional ByVal b As Double = 1.2, _
        Optional ByVal c As Integer = 1)
        
        Console.WriteLine("a = " & a & " b = " & b & " c = " & c)
    End Sub
End Class

Class Example
    Public Shared Sub Main()
        Dim o As New OptionalArg()
        Dim t As Type
        t = GetType(OptionalArg)

        Dim bf As BindingFlags = _
            BindingFlags.Public Or BindingFlags.Instance Or _
            BindingFlags.InvokeMethod Or BindingFlags.OptionalParamBinding

        t.InvokeMember("MyMethod", bf, Nothing, o, New Object() {10, 55.3, 12})
        t.InvokeMember("MyMethod", bf, Nothing, o, New Object() {10, 1.3, Type.Missing})
        t.InvokeMember("MyMethod", bf, Nothing, o, New Object() {10, Type.Missing, Type.Missing})
    End Sub
End Class

Mit diesem Code wird die folgende Ausgabe generiert:

a = 10 b = 55,3 c = 12

a = 10 b = 1,3 c = 1

a = 10 b = 1,2 c = 1

Hinweise

Verwenden Sie das Missing Feld zum Aufrufen durch Spiegelung, um den Standardwert eines Parameters abzurufen. Wenn das Missing Feld für einen Parameterwert übergeben wird und für diesen Parameter kein Standardwert vorhanden ist, wird ein ArgumentException Fehler ausgelöst.

Weitere Informationen

• Missing