GenericTypeParameterBuilder.SetBaseTypeConstraint(Type) 方法

定義

命名空間:

System.Reflection.Emit

組件:

netstandard.dll, System.Reflection.Emit.dll

來源:

GenericTypeParameterBuilder.cs

設定基底類型必須繼承才能取代成類型參數的類型。

C#

public void SetBaseTypeConstraint(Type? baseTypeConstraint);

參數

baseTypeConstraint

Type

所有類型必須繼承才能取代成型別參數的 Type。

範例

下列程式代碼範例會建立具有兩個型別參數的泛型型別，第二個型別具有基底類型條件約束，並將它們儲存在元件 GenericEmitExample1.dll 中。 您可以使用 Ildasm.exe (IL 反組譯程式) 來檢視產生的類型。 如需定義動態泛型類型相關步驟的更詳細說明，請參閱 如何：使用反映發出定義泛型類型。

C#

using System;
using System.Reflection;
using System.Reflection.Emit;
using System.Collections.Generic;

// Define a trivial base class and two trivial interfaces
// to use when demonstrating constraints.
//
public class ExampleBase {}

public interface IExampleA {}

public interface IExampleB {}

// Define a trivial type that can substitute for type parameter
// TSecond.
//
public class ExampleDerived : ExampleBase, IExampleA, IExampleB {}

public class Example
{
    public static void Main()
    {
        // Define a dynamic assembly to contain the sample type. The
        // assembly will not be run, but only saved to disk, so
        // AssemblyBuilderAccess.Save is specified.
        //
        AppDomain myDomain = AppDomain.CurrentDomain;
        AssemblyName myAsmName = new AssemblyName("GenericEmitExample1");
        AssemblyBuilder myAssembly =
            myDomain.DefineDynamicAssembly(myAsmName,
                AssemblyBuilderAccess.RunAndSave);

        // An assembly is made up of executable modules. For a single-
        // module assembly, the module name and file name are the same
        // as the assembly name.
        //
        ModuleBuilder myModule =
            myAssembly.DefineDynamicModule(myAsmName.Name,
               myAsmName.Name + ".dll");

        // Get type objects for the base class trivial interfaces to
        // be used as constraints.
        //
        Type baseType = typeof(ExampleBase);
        Type interfaceA = typeof(IExampleA);
        Type interfaceB = typeof(IExampleB);

        // Define the sample type.
        //
        TypeBuilder myType =
            myModule.DefineType("Sample", TypeAttributes.Public);

        Console.WriteLine("Type 'Sample' is generic: {0}",
            myType.IsGenericType);

        // Define type parameters for the type. Until you do this,
        // the type is not generic, as the preceding and following
        // WriteLine statements show. The type parameter names are
        // specified as an array of strings. To make the code
        // easier to read, each GenericTypeParameterBuilder is placed
        // in a variable with the same name as the type parameter.
        //
        string[] typeParamNames = {"TFirst", "TSecond"};
        GenericTypeParameterBuilder[] typeParams =
            myType.DefineGenericParameters(typeParamNames);

        GenericTypeParameterBuilder TFirst = typeParams[0];
        GenericTypeParameterBuilder TSecond = typeParams[1];

        Console.WriteLine("Type 'Sample' is generic: {0}",
            myType.IsGenericType);

        // Apply constraints to the type parameters.
        //
        // A type that is substituted for the first parameter, TFirst,
        // must be a reference type and must have a parameterless
        // constructor.
        TFirst.SetGenericParameterAttributes(
            GenericParameterAttributes.DefaultConstructorConstraint |
            GenericParameterAttributes.ReferenceTypeConstraint);

        // A type that is substituted for the second type
        // parameter must implement IExampleA and IExampleB, and
        // inherit from the trivial test class ExampleBase. The
        // interface constraints are specified as an array
        // containing the interface types.
        TSecond.SetBaseTypeConstraint(baseType);
        Type[] interfaceTypes = {interfaceA, interfaceB};
        TSecond.SetInterfaceConstraints(interfaceTypes);

        // The following code adds a private field named ExampleField,
        // of type TFirst.
        FieldBuilder exField =
            myType.DefineField("ExampleField", TFirst,
                FieldAttributes.Private);

        // Define a static method that takes an array of TFirst and
        // returns a List<TFirst> containing all the elements of
        // the array. To define this method it is necessary to create
        // the type List<TFirst> by calling MakeGenericType on the
        // generic type definition, List<T>. (The T is omitted with
        // the typeof operator when you get the generic type
        // definition.) The parameter type is created by using the
        // MakeArrayType method.
        //
        Type listOf = typeof(List<>);
        Type listOfTFirst = listOf.MakeGenericType(TFirst);
        Type[] mParamTypes = {TFirst.MakeArrayType()};

        MethodBuilder exMethod =
            myType.DefineMethod("ExampleMethod",
                MethodAttributes.Public | MethodAttributes.Static,
                listOfTFirst,
                mParamTypes);

        // Emit the method body.
        // The method body consists of just three opcodes, to load
        // the input array onto the execution stack, to call the
        // List<TFirst> constructor that takes IEnumerable<TFirst>,
        // which does all the work of putting the input elements into
        // the list, and to return, leaving the list on the stack. The
        // hard work is getting the constructor.
        //
        // The GetConstructor method is not supported on a
        // GenericTypeParameterBuilder, so it is not possible to get
        // the constructor of List<TFirst> directly. There are two
        // steps, first getting the constructor of List<T> and then
        // calling a method that converts it to the corresponding
        // constructor of List<TFirst>.
        //
        // The constructor needed here is the one that takes an
        // IEnumerable<T>. Note, however, that this is not the
        // generic type definition of IEnumerable<T>; instead, the
        // T from List<T> must be substituted for the T of
        // IEnumerable<T>. (This seems confusing only because both
        // types have type parameters named T. That is why this example
        // uses the somewhat silly names TFirst and TSecond.) To get
        // the type of the constructor argument, take the generic
        // type definition IEnumerable<T> (expressed as
        // IEnumerable<> when you use the typeof operator) and
        // call MakeGenericType with the first generic type parameter
        // of List<T>. The constructor argument list must be passed
        // as an array, with just one argument in this case.
        //
        // Now it is possible to get the constructor of List<T>,
        // using GetConstructor on the generic type definition. To get
        // the constructor of List<TFirst>, pass List<TFirst> and
        // the constructor from List<T> to the static
        // TypeBuilder.GetConstructor method.
        //
        ILGenerator ilgen = exMethod.GetILGenerator();

        Type ienumOf = typeof(IEnumerable<>);
        Type TfromListOf = listOf.GetGenericArguments()[0];
        Type ienumOfT = ienumOf.MakeGenericType(TfromListOf);
        Type[] ctorArgs = {ienumOfT};

        ConstructorInfo ctorPrep = listOf.GetConstructor(ctorArgs);
        ConstructorInfo ctor =
            TypeBuilder.GetConstructor(listOfTFirst, ctorPrep);

        ilgen.Emit(OpCodes.Ldarg_0);
        ilgen.Emit(OpCodes.Newobj, ctor);
        ilgen.Emit(OpCodes.Ret);

        // Create the type and save the assembly.
        Type finished = myType.CreateType();
        myAssembly.Save(myAsmName.Name+".dll");

        // Invoke the method.
        // ExampleMethod is not generic, but the type it belongs to is
        // generic, so in order to get a MethodInfo that can be invoked
        // it is necessary to create a constructed type. The Example
        // class satisfies the constraints on TFirst, because it is a
        // reference type and has a default constructor. In order to
        // have a class that satisfies the constraints on TSecond,
        // this code example defines the ExampleDerived type. These
        // two types are passed to MakeGenericMethod to create the
        // constructed type.
        //
        Type[] typeArgs = {typeof(Example), typeof(ExampleDerived)};
        Type constructed = finished.MakeGenericType(typeArgs);
        MethodInfo mi = constructed.GetMethod("ExampleMethod");

        // Create an array of Example objects, as input to the generic
        // method. This array must be passed as the only element of an
        // array of arguments. The first argument of Invoke is
        // null, because ExampleMethod is static. Display the count
        // on the resulting List<Example>.
        //
        Example[] input = {new Example(), new Example()};
        object[] arguments = {input};

        List<Example> listX =
            (List<Example>) mi.Invoke(null, arguments);

        Console.WriteLine(
            "\nThere are {0} elements in the List<Example>.",
            listX.Count);

        DisplayGenericParameters(finished);
    }

    private static void DisplayGenericParameters(Type t)
    {
        if (!t.IsGenericType)
        {
            Console.WriteLine("Type '{0}' is not generic.");
            return;
        }
        if (!t.IsGenericTypeDefinition)
        {
            t = t.GetGenericTypeDefinition();
        }

        Type[] typeParameters = t.GetGenericArguments();
        Console.WriteLine("\nListing {0} type parameters for type '{1}'.",
            typeParameters.Length, t);

        foreach( Type tParam in typeParameters )
        {
            Console.WriteLine("\r\nType parameter {0}:", tParam.ToString());

            foreach( Type c in tParam.GetGenericParameterConstraints() )
            {
                if (c.IsInterface)
                {
                    Console.WriteLine("    Interface constraint: {0}", c);
                }
                else
                {
                    Console.WriteLine("    Base type constraint: {0}", c);
                }
            }

            ListConstraintAttributes(tParam);
        }
    }

    // List the constraint flags. The GenericParameterAttributes
    // enumeration contains two sets of attributes, variance and
    // constraints. For this example, only constraints are used.
    //
    private static void ListConstraintAttributes(Type t)
    {
        // Mask off the constraint flags.
        GenericParameterAttributes constraints =
            t.GenericParameterAttributes & GenericParameterAttributes.SpecialConstraintMask;

        if ((constraints & GenericParameterAttributes.ReferenceTypeConstraint)
            != GenericParameterAttributes.None)
        {
            Console.WriteLine("    ReferenceTypeConstraint");
        }

        if ((constraints & GenericParameterAttributes.NotNullableValueTypeConstraint)
            != GenericParameterAttributes.None)
        {
            Console.WriteLine("    NotNullableValueTypeConstraint");
        }

        if ((constraints & GenericParameterAttributes.DefaultConstructorConstraint)
            !=GenericParameterAttributes.None)
        {
            Console.WriteLine("    DefaultConstructorConstraint");
        }
    }
}

/* This code example produces the following output:

Type 'Sample' is generic: False
Type 'Sample' is generic: True

There are 2 elements in the List<Example>.

Listing 2 type parameters for type 'Sample[TFirst,TSecond]'.

Type parameter TFirst:
    ReferenceTypeConstraint
    DefaultConstructorConstraint

Type parameter TSecond:
    Interface constraint: IExampleA
    Interface constraint: IExampleB
    Base type constraint: ExampleBase
 */

備註

若要擷取基底類型條件約束，請使用 BaseType 屬性。

建立包含型別參數的泛型型別之後，您可以使用其 Type 物件來反映型別參數及其條件約束。 若要取得已完成泛型型別的類型參數，請使用 Type.GetGenericArguments 方法。 針對每個類型參數，使用 Type.GetGenericParameterConstraints 方法取得基底類型條件約束和介面條件約束，並使用 屬性取得特殊條件約束 Type.GenericParameterAttributes 。

適用於

另請參閱

• 作法：使用反映發出定義泛型型別