Type.Missing Champ
Important
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Représente une valeur manquante dans les informations Type. Ce champ est en lecture seule.
public: static initonly System::Object ^ Missing;C#
public static readonly object Missing; staticval mutable Missing : objPublic Shared ReadOnly Missing As Object L’exemple de code suivant illustre l’utilisation du Missing champ pour appeler une méthode avec ses arguments par défaut.
#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();
}
C#
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
Ce code génère la sortie suivante :
a = 10 b = 55,3 c = 12
a = 10 b = 1,3 c = 1
a = 10 b = 1,2 c = 1
Utilisez le Missing champ pour l’appel via la réflexion pour obtenir la valeur par défaut d’un paramètre. Si le Missing champ est passé pour une valeur de paramètre et qu’il n’existe pas de valeur par défaut pour ce paramètre, une ArgumentException exception est levée.
| Produit | Versions |
|---|---|
| .NET | Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7 |
| .NET Framework | 1.1, 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8 |
| .NET Standard | 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 2.0, 2.1 |
| UWP | 10.0 |