如何使用反射来分析C#中特殊类型的约束？

Question

请参考以下代码：

public class GenericTest2
{

    public class MyGenericClass<T, U, V, W>
        where T : class
        where U : new()
        where V : struct
        where W : System.IO.StringWriter
    {
    }

    public static void Test()
    {
        Assembly a = Assembly.GetAssembly(typeof(GenericTest));
        foreach (Type t in a.GetTypes()) {
            Console.Out.WriteLine(t.FullName);
            if (t.IsGenericType) {
                Console.Out.WriteLine("\tIsGeneric!");
                foreach (Type parm in t.GetGenericArguments()) {
                    Console.Out.WriteLine("\tGeneric parameter: " + parm.Name);
                    Type[] constraints = parm.GetGenericParameterConstraints();
                    for (int i = 0; i < constraints.Length; i++) {
                        Console.Out.WriteLine("\t\t constraint " + i + ": name = " + constraints[i].Name);
                        Console.Out.WriteLine("\t\t constraint " + i + ": fullname = " + constraints[i].FullName);
                    }
                }
            }
        }

    }

}

这段代码将输出如下所示：

ProcessCSharpAssemblies.Program
ProcessCSharpAssemblies.GenericTest2
ProcessCSharpAssemblies.GenericTest2+MyGenericClass`4
    IsGeneric!
    Generic parameter: T
    Generic parameter: U
    Generic parameter: V
        constraint 0: name = ValueType
        constraint 0: fullname = System.ValueType
    Generic parameter: W
        constraint 0: name = StringWriter
        constraint 0: fullname = System.IO.StringWriter

约束class和new()似乎未由parm.GetGenericParameterConstraints()返回。尽管存在对T和U的约束，但parm.GetGenericParameterConstraints()并不返回数据。

问:如何使用反射检查这些类型的约束？

Answer 1

您正在寻找的约束可以通过查看Type的GenericParameterAttributes来找到。

var gpa = parm.GenericParameterAttributes;

var constraints = gpa & GenericParameterAttributes.SpecialConstraintMask;

if ((constraints & GenericParameterAttributes.DefaultConstructorConstraint) != 0)
{
    // yippie!
}