Type.Missing Campo

Definición

Representa un valor que falta en la información de Type.Represents a missing value in the Type information. Este campo es de solo lectura.This field is read-only.

public: static initonly System::Object ^ Missing;
public static readonly object Missing;
 staticval mutable Missing : obj
Public Shared ReadOnly Missing As Object 

Valor de campo

Ejemplos

En el ejemplo de código siguiente se muestra el uso del campo Missing para invocar un método con sus argumentos predeterminados.The following code example shows the use of the Missing field to invoke a method with its default arguments.

#using <System.dll>

using namespace System;
using namespace System::Reflection;
using namespace System::CodeDom::Compiler;

ref 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, nullptr, o, gcnew array<Object^> {10, 55.3, 12});
        t->InvokeMember("MyMethod", bf, nullptr, o, gcnew array<Object^> {10, 1.3, Type::Missing});
        t->InvokeMember("MyMethod", bf, nullptr, o, gcnew array<Object^> {10, Type::Missing, Type::Missing});
    }

private:
    static Object^ GenerateObjectFromSource(String^ objectName,
        String^ sourceLines, String^ providerName)
    {
        Object^ genObject = nullptr;
        CodeDomProvider^ codeProvider = CodeDomProvider::CreateProvider(providerName);
        CompilerParameters^ cp = gcnew 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;
    }
};

int main()
{
    Example::Main();
}
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;
    }
}
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

Este código genera el siguiente resultado:This code produces the following output:

a = 10 b = 55,3 c = 12a = 10 b = 55.3 c = 12

a = 10 b = 1,3 c = 1a = 10 b = 1.3 c = 1

a = 10 b = 1,2 c = 1a = 10 b = 1.2 c = 1

Comentarios

Use el campo Missing para la invocación a través de la reflexión para obtener el valor predeterminado de un parámetro.Use the Missing field for invocation through reflection to obtain the default value of a parameter. Si se pasa el campo Missing para un valor de parámetro y no hay ningún valor predeterminado para ese parámetro, se produce una ArgumentException.If the Missing field is passed in for a parameter value and there is no default value for that parameter, an ArgumentException is thrown.

Se aplica a

Consulte también: