如何在无法事先知道方法签名的情况下从MethodInfo创建委托

Question

我需要一个方法，它接受一个具有任意签名的非泛型静态方法的MethodInfo实例，并返回一个绑定到该方法的委托，该方法可以稍后使用Delegate.DynamicInvoke方法调用。我的第一个naïve尝试看起来像这样:

using System;
using System.Reflection;
class Program
{
    static void Main()
    {
        var method = CreateDelegate(typeof (Console).GetMethod("WriteLine", new[] {typeof (string)}));
        method.DynamicInvoke("Hello world");
    }
    static Delegate CreateDelegate(MethodInfo method)
    {
        if (method == null)
        {
            throw new ArgumentNullException("method");
        }
        if (!method.IsStatic)
        {
            throw new ArgumentNullException("method", "The provided method is not static.");
        }
        if (method.ContainsGenericParameters)
        {
            throw new ArgumentException("The provided method contains unassigned generic type parameters.");
        }
        return method.CreateDelegate(typeof(Delegate)); // This does not work: System.ArgumentException: Type must derive from Delegate.
    }
}

我希望MethodInfo.CreateDelegate方法能够自己找出正确的委托类型。显然不能。那么，如何创建一个System.Type的实例来表示具有与所提供的MethodInfo实例匹配的签名的委托呢?

Answer 1

可以使用System.Linq.Expressions.Expression.GetDelegateType方法:

using System;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
class Program
{
    static void Main()
    {
        var writeLine = CreateDelegate(typeof(Console).GetMethod("WriteLine", new[] { typeof(string) }));
        writeLine.DynamicInvoke("Hello world");
        var readLine = CreateDelegate(typeof(Console).GetMethod("ReadLine", Type.EmptyTypes));
        writeLine.DynamicInvoke(readLine.DynamicInvoke());
    }
    static Delegate CreateDelegate(MethodInfo method)
    {
        if (method == null)
        {
            throw new ArgumentNullException("method");
        }
        if (!method.IsStatic)
        {
            throw new ArgumentException("The provided method must be static.", "method");
        }
        if (method.IsGenericMethod)
        {
            throw new ArgumentException("The provided method must not be generic.", "method");
        }
        return method.CreateDelegate(Expression.GetDelegateType(
            (from parameter in method.GetParameters() select parameter.ParameterType)
            .Concat(new[] { method.ReturnType })
            .ToArray()));
    }
}

在!method.IsStatic的第二次检查中可能存在复制粘贴错误-您不应该在那里使用ArgumentNullException。提供参数名作为ArgumentException的参数是一种很好的样式。

如果您想拒绝所有泛型方法，请使用method.IsGenericMethod;如果您只想拒绝具有未替换类型参数的泛型方法，请使用method.ContainsGenericParameters。

Answer 2

你可以试试System.LinQ.Expressions

...
using System.Linq.Expressions;
...
static Delegate CreateMethod(MethodInfo method)
{
    if (method == null)
    {
        throw new ArgumentNullException("method");
    }
    if (!method.IsStatic)
    {
        throw new ArgumentException("The provided method must be static.", "method");
    }
    if (method.IsGenericMethod)
    {
        throw new ArgumentException("The provided method must not be generic.", "method");
    }
    var parameters = method.GetParameters()
                           .Select(p => Expression.Parameter(p.ParameterType, p.Name))
                           .ToArray();
    var call = Expression.Call(null, method, parameters);
    return Expression.Lambda(call, parameters).Compile();
}

并在后面使用

var method = CreateMethod(typeof (Console).GetMethod("WriteLine", new[] {typeof (string)}));
method.DynamicInvoke("Test Test");

Answer 3

如果你想创建一个非静态方法，你必须实现一个目标/实例对象。

/// <summary>
/// Create delegate by methodinfo in target
/// </summary>
/// <param name="method">method info</param>
/// <param name="target">A instance of the object which contains the method where will be execute</param>
/// <returns>delegate or null</returns>
public static Delegate? CreateDelegateWithTarget(MethodInfo? method, object? target)
{
    if (method is null ||
        target is null)
        return null;
    if (method.IsStatic)
        return null;
    if (method.IsGenericMethod)
        return null;
    return method.CreateDelegate(Expression.GetDelegateType(
        (from parameter in method.GetParameters() select parameter.ParameterType)
        .Concat(new[] { method.ReturnType })
        .ToArray()), target);
}