Type.IsAssignableFrom(Type) Metoda

public:
 virtual bool IsAssignableFrom(Type ^ c);

public virtual bool IsAssignableFrom(Type? c);

public virtual bool IsAssignableFrom(Type c);

abstract member IsAssignableFrom : Type -> bool
override this.IsAssignableFrom : Type -> bool

Public Overridable Function IsAssignableFrom (c As Type) As Boolean

Parametry

Návraty

true pokud je splněna některá z následujících podmínek:

• c a aktuální instance představují stejný typ.

• c je odvozen přímo nebo nepřímo z aktuální instance. c je odvozen přímo z aktuální instance, pokud dědí z aktuální instance; c je odvozen nepřímo z aktuální instance, pokud dědí z posloupnosti jedné nebo více tříd, které dědí z aktuální instance.

• Aktuální instance je rozhraní, které c implementuje.

• c je parametr obecného typu a aktuální instance představuje jednu z omezení c.

• c představuje typ hodnoty a aktuální instance představuje Nullable<c> (Nullable(Of c) v Visual Basic).

false pokud žádná z těchto podmínek není pravdivá nebo pokud c je null.

Implementuje

Příklady

Následující příklad ukazuje metodu IsAssignableFrom pomocí definovaných tříd, celočíselná pole a obecné typy.

using System;
using System.Collections.Generic;
class Program
{
    public static void Main()
    {
            // Demonstrate classes:
            Console.WriteLine("Defined Classes:");
            Room room1 = new Room();
            Kitchen kitchen1 = new Kitchen();
            Bedroom bedroom1 = new Bedroom();
            Guestroom guestroom1 = new Guestroom();
            MasterBedroom masterbedroom1 = new MasterBedroom();

            Type room1Type = room1.GetType();
            Type kitchen1Type = kitchen1.GetType();
            Type bedroom1Type = bedroom1.GetType();
            Type guestroom1Type = guestroom1.GetType();
            Type masterbedroom1Type = masterbedroom1.GetType();

            Console.WriteLine("room assignable from kitchen: {0}", room1Type.IsAssignableFrom(kitchen1Type));
            Console.WriteLine("bedroom assignable from guestroom: {0}", bedroom1Type.IsAssignableFrom(guestroom1Type));
            Console.WriteLine("kitchen assignable from masterbedroom: {0}", kitchen1Type.IsAssignableFrom(masterbedroom1Type));

            // Demonstrate arrays:
            Console.WriteLine();
            Console.WriteLine("Integer arrays:");

            int[] array2 = new int[2];
            int[] array10 = new int[10];
            int[,] array22 = new int[2, 2];
            int[,] array24 = new int[2, 4];

            Type array2Type = array2.GetType();
            Type array10Type = array10.GetType();
            Type array22Type = array22.GetType();
            Type array24Type = array24.GetType();

            Console.WriteLine("int[2] assignable from int[10]: {0}", array2Type.IsAssignableFrom(array10Type));
            Console.WriteLine("int[2] assignable from int[2,4]: {0}", array2Type.IsAssignableFrom(array24Type));
            Console.WriteLine("int[2,4] assignable from int[2,2]: {0}", array24Type.IsAssignableFrom(array22Type));

            // Demonstrate generics:
            Console.WriteLine();
            Console.WriteLine("Generics:");

            // Note that "int?[]" is the same as "Nullable<int>[]"
            int?[] arrayNull = new int?[10];
            List<int> genIntList = new List<int>();
            List<Type> genTList = new List<Type>();

            Type arrayNullType = arrayNull.GetType();
            Type genIntListType = genIntList.GetType();
            Type genTListType = genTList.GetType();

            Console.WriteLine("int[10] assignable from int?[10]: {0}", array10Type.IsAssignableFrom(arrayNullType));
            Console.WriteLine("List<int> assignable from List<Type>: {0}", genIntListType.IsAssignableFrom(genTListType));
            Console.WriteLine("List<Type> assignable from List<int>: {0}", genTListType.IsAssignableFrom(genIntListType));

            Console.ReadLine();
    }
}
class Room
{
}

class Kitchen : Room
{
}

class Bedroom : Room
{
}

class Guestroom : Bedroom
{
}

class MasterBedroom : Bedroom
{
}

//This code example produces the following output:
//
// Defined Classes:
// room assignable from kitchen: True
// bedroom assignable from guestroom: True
// kitchen assignable from masterbedroom: False
//
// Integer arrays:
// int[2] assignable from int[10]: True
// int[2] assignable from int[2,4]: False
// int[2,4] assignable from int[2,2]: True
//
// Generics:
// int[10] assignable from int?[10]: False
// List<int> assignable from List<Type>: False
// List<Type> assignable from List<int>: False

open System

type Room() = class end

type Kitchen() = inherit Room()

type Bedroom() = inherit Room()

type Guestroom() = inherit Bedroom()

type MasterBedroom() = inherit Bedroom()

// Demonstrate classes:
printfn "Defined Classes:"
let room1 = Room()
let kitchen1 = Kitchen()
let bedroom1 = Bedroom()
let guestroom1 = Guestroom()
let masterbedroom1 = MasterBedroom()

let room1Type = room1.GetType()
let kitchen1Type = kitchen1.GetType()
let bedroom1Type = bedroom1.GetType()
let guestroom1Type = guestroom1.GetType()
let masterbedroom1Type = masterbedroom1.GetType()

printfn $"room assignable from kitchen: {room1Type.IsAssignableFrom kitchen1Type}"
printfn $"bedroom assignable from guestroom: {bedroom1Type.IsAssignableFrom guestroom1Type}"
printfn $"kitchen assignable from masterbedroom: {kitchen1Type.IsAssignableFrom masterbedroom1Type}"

// Demonstrate arrays:
printfn "\nInteger arrays:"

let array2 = Array.zeroCreate<int> 2
let array10 = Array.zeroCreate<int> 10
let array22 = Array2D.zeroCreate<int> 2 2
let array24 = Array2D.zeroCreate<int> 2 4

let array2Type = array2.GetType()
let array10Type = array10.GetType()
let array22Type = array22.GetType()
let array24Type = array24.GetType()

printfn $"int[2] assignable from int[10]: {array2Type.IsAssignableFrom array10Type}"
printfn $"int[2] assignable from int[2,4]: {array2Type.IsAssignableFrom array24Type}"
printfn $"int[2,4] assignable from int[2,2]: {array24Type.IsAssignableFrom array22Type}"

// Demonstrate generics:
printfn "\nGenerics:"

let arrayNull = Array.zeroCreate<Nullable<int>> 10
let genIntList = ResizeArray<int>()
let genTList = ResizeArray<Type>()

let arrayNullType = arrayNull.GetType()
let genIntListType = genIntList.GetType()
let genTListType = genTList.GetType()

printfn $"int[10] assignable from int?[10]: {array10Type.IsAssignableFrom arrayNullType}"
printfn $"List<int> assignable from List<Type>: {genIntListType.IsAssignableFrom genTListType}"
printfn $"List<Type> assignable from List<int>: {genTListType.IsAssignableFrom genIntListType}"

//This code example produces the following output:
//
// Defined Classes:
// room assignable from kitchen: True
// bedroom assignable from guestroom: True
// kitchen assignable from masterbedroom: False
//
// Integer arrays:
// int[2] assignable from int[10]: True
// int[2] assignable from int[2,4]: False
// int[2,4] assignable from int[2,2]: True
//
// Generics:
// int[10] assignable from int?[10]: False
// List<int> assignable from List<Type>: False
// List<Type> assignable from List<int>: False

Imports System.Collections.Generic

Module Example
    Public Sub Main()
        Console.WriteLine("Defined Classes:")
        Dim room1 As New Room()
        Dim kitchen1 As New Kitchen()
        Dim bedroom1 As New Bedroom()
        Dim guestroom1 As New Guestroom()
        Dim masterbedroom1 As New MasterBedroom()

        Dim room1Type As Type = room1.GetType()
        Dim kitchen1Type As Type = kitchen1.GetType()
        Dim bedroom1Type As Type = bedroom1.GetType()
        Dim guestroom1Type As Type = guestroom1.GetType()
        Dim masterbedroom1Type As Type = masterbedroom1.GetType()

        Console.WriteLine("room assignable from kitchen: {0}", room1Type.IsAssignableFrom(kitchen1Type))
        Console.WriteLine("bedroom assignable from guestroom: {0}", bedroom1Type.IsAssignableFrom(guestroom1Type))
        Console.WriteLine("kitchen assignable from masterbedroom: {0}", kitchen1Type.IsAssignableFrom(masterbedroom1Type))

        ' Demonstrate arrays:
        Console.WriteLine()
        Console.WriteLine("Integer arrays:")

        Dim array10(10) As Integer
        Dim array2(2) As Integer
        Dim array22(2, 2) As Integer
        Dim array24(2, 4) As Integer

        Dim array10Type As Type = array10.GetType
        Dim array2Type As Type = array2.GetType
        Dim array22Type As Type = array22.GetType
        Dim array24Type As Type = array24.GetType

        Console.WriteLine("Integer(2) assignable from Integer(10): {0}", array2Type.IsAssignableFrom(array10Type))
        Console.WriteLine("Integer(2) assignable from Integer(2,4): {0}", array2Type.IsAssignableFrom(array24Type))
        Console.WriteLine("Integer(2,4) assignable from Integer(2,2): {0}", array24Type.IsAssignableFrom(array22Type))

        ' Demonstrate generics:
        Console.WriteLine()
        Console.WriteLine("Generics:")

        Dim arrayNull(10) As Nullable(Of Integer)
        Dim genIntList As New List(Of Integer)
        Dim genTList As New List(Of Type)

        Dim arrayNullType As Type = arrayNull.GetType
        Dim genIntListType As Type = genIntList.GetType
        Dim genTListType As Type = genTList.GetType

        Console.WriteLine("Integer(10) assignable from Nullable(Of Integer)(10): {0}", array10Type.IsAssignableFrom(arrayNullType))
        Console.WriteLine("List(Of Integer) assignable from List(Of Type): {0}", genIntListType.IsAssignableFrom(genTListType))
        Console.WriteLine("List(Of Type) assignable from List(Of Integer): {0}", genTListType.IsAssignableFrom(genIntListType))
        Console.ReadLine()
    End Sub
End Module

Class Room
End Class

Class Kitchen : Inherits Room
End Class

Class Bedroom : Inherits Room
End Class

Class Guestroom : Inherits Bedroom
End Class

Class MasterBedroom : Inherits Bedroom
End Class
' The example displays the following output:
'    Defined Classes:
'    room assignable from kitchen: True
'    bedroom assignable from guestroom: True
'    kitchen assignable from masterbedroom: False
'
'    Integer arrays:
'    Integer(2) assignable from Integer(10): True
'    Integer(2) assignable from Integer(2,4): False
'    Integer(2,4) assignable from Integer(2,2): True
'
'    Generics:
'    Integer(10) assignable from Nullable(Of Integer)(10): False
'    List(Of Integer) assignable from List(Of Type): False
'    List(Of Type) assignable from List(Of Integer): False

V následujícím příkladu je Type aktuální instance objekt, který představuje Stream třídu. GenericWithConstraint je obecný typ, jehož parametr obecného typu musí být typu Stream. Předání parametru IsAssignableFrom obecného typu metodě označuje, že instanci parametru obecného typu lze přiřadit k objektu Stream .

using System;
using System.IO;

public class Example
{
   public static void Main()
   {
      Type t = typeof(Stream);
      Type genericT = typeof(GenericWithConstraint<>);
      Type genericParam = genericT.GetGenericArguments()[0];
      Console.WriteLine(t.IsAssignableFrom(genericParam));  
      // Displays True.
   }
}

public class GenericWithConstraint<T> where T : Stream
{}

open System.IO

type GenericWithConstraint<'T when 'T :> Stream>() = class end

let t = typeof<Stream>
let genericT = typeof<GenericWithConstraint<_>>.GetGenericTypeDefinition()
let genericParam = genericT.GetGenericArguments()[0]
printfn $"{t.IsAssignableFrom genericParam}"
// Displays True.

Imports System.IO

Module Example
   Public Sub Main()
      Dim t As Type = GetType(Stream)
      Dim genericT As Type = GetType(GenericWithConstraint(Of ))
      Dim genericParam As Type = genericT.GetGenericArguments()(0)
      Console.WriteLine(t.IsAssignableFrom(genericParam))  
      ' Displays True.
   End Sub
End Module

Public Class GenericWithConstraint(Of T As Stream)
End Class

Poznámky

Metodu IsAssignableFrom lze použít k určení, zda je možné přiřadit instanci c aktuálního typu, Metoda je nejužitečnější při zpracování objektů, jejichž typy nejsou známy v době návrhu a umožňují podmíněné přiřazení, jak ukazuje následující příklad.

using System;
using System.Collections;

public class Example
{
   public static void Main()
   {
      Type t = typeof(IEnumerable);
      Type c = typeof(Array);
      
      IEnumerable instanceOfT;
      int[] instanceOfC = { 1, 2, 3, 4 };
      if (t.IsAssignableFrom(c))
         instanceOfT = instanceOfC;
  }
}

open System
open System.Collections

let t = typeof<IEnumerable>
let c = typeof<Array>

let mutable instanceOfT = Unchecked.defaultof<IEnumerable>
let instanceOfC = [| 1; 2; 3; 4 |]
if t.IsAssignableFrom c then
    instanceOfT <- instanceOfC

Imports System.Collections

Module Example
   Public Sub Main()
      Dim t As Type = GetType(IEnumerable)
      Dim c As Type = GetType(Array)
      
      Dim instanceOfT As IEnumerable
      Dim instanceOfC As Integer() = { 1, 2, 3, 4 }
      If t.IsAssignableFrom(c) Then
         instanceOfT = instanceOfC
      End If  
   End Sub
End Module

Tato metoda tak zajistí, že se řádek kódu podobný následujícímu spustí za běhu bez vyvolání InvalidCastException výjimky nebo podobné výjimky:

instanceOfT = instanceOfC;

instanceOfT <- instanceOfC

instanceOfT = instanceOfC

Tuto metodu lze přepsat odvozenou třídou.

c Pokud je parametr typuTypeBuilder, výsledek je založený na typu, který se má sestavit. Následující příklad kódu ukazuje použití sestaveného typu s názvem B.

using System;
using System.Reflection;
using System.Reflection.Emit;

public class A
{}

public class Example
{
   public static void Main()
   {
      AppDomain domain = AppDomain.CurrentDomain;
      AssemblyName assemName = new AssemblyName();
      assemName.Name = "TempAssembly";

      // Define a dynamic assembly in the current application domain.
      AssemblyBuilder assemBuilder = domain.DefineDynamicAssembly(assemName,
                                            AssemblyBuilderAccess.Run);

      // Define a dynamic module in this assembly.
      ModuleBuilder moduleBuilder = assemBuilder.DefineDynamicModule("TempModule");

      TypeBuilder b1 = moduleBuilder.DefineType("B", TypeAttributes.Public, typeof(A));
      Console.WriteLine(typeof(A).IsAssignableFrom(b1));
   }
}
// The example displays the following output:
//        True

open System
open System.Reflection
open System.Reflection.Emit

type A() = class end

let domain = AppDomain.CurrentDomain
let assemName = AssemblyName()
assemName.Name <- "TempAssembly"

// Define a dynamic assembly in the current application domain.
let assemBuilder = 
    domain.DefineDynamicAssembly(assemName, AssemblyBuilderAccess.Run)

// Define a dynamic module in this assembly.
let moduleBuilder = 
    assemBuilder.DefineDynamicModule "TempModule"

let b1 = moduleBuilder.DefineType("B", TypeAttributes.Public, typeof<A>)
printfn $"{typeof<A>.IsAssignableFrom b1}"
// The example displays the following output:
//        True

Imports System.Reflection
Imports System.Reflection.Emit

Public Class A
End Class

Module Example
   Public Sub Main()
      Dim domain As AppDomain = AppDomain.CurrentDomain
      Dim assemName As New AssemblyName()
      assemName.Name = "TempAssembly"

      ' Define a dynamic assembly in the current application domain.
      Dim assemBuilder As AssemblyBuilder = domain.DefineDynamicAssembly(assemName,
                                                   AssemblyBuilderAccess.Run)

      ' Define a dynamic module in this assembly.
      Dim moduleBuilder As ModuleBuilder = assemBuilder.DefineDynamicModule("TempModule")

      Dim b1 As TypeBuilder = moduleBuilder.DefineType("B", TypeAttributes.Public, GetType(A))
      Console.WriteLine(GetType(A).IsAssignableFrom(b1))
   End Sub
End Module
' The example displays the following output:
'       True