Type.Missing Pole

Definice

Obor názvů:

System

Sestavení:

netstandard.dll, System.Runtime.dll

Zdroj:

Type.cs

Představuje chybějící hodnotu v informacích Type . Toto pole je určeno pouze ke čtení.

public: static initonly System::Object ^ Missing;

public static readonly object Missing;

 staticval mutable Missing : obj

Public Shared ReadOnly Missing As Object 

Hodnota pole

Object

Příklady

Následující příklad kódu ukazuje použití Missing pole k vyvolání metody s výchozími argumenty.

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

Tento kód vytvoří následující výstup:

a = 10 b = 55,3 c = 12

a = 10 b = 1,3 c = 1

a = 10 b = 1,2 c = 1

Poznámky

Missing Pomocí pole pro vyvolání prostřednictvím reflexe získejte výchozí hodnotu parametru. Missing Pokud je pole předáno pro hodnotu parametru a neexistuje žádná výchozí hodnota pro tento parametr, ArgumentException vyvolá se.

Viz také

• Missing