Generic constructors and reflection

Question

Is it possible to see which constructor was the generic one?

internal class Foo
{
  public Foo( T value ) {}
  public Foo( string value ) {}
}

var cons         


        

Answer 1

You want System.Reflection.ParameterInfo.ParameterType.IsGenericParameter. Here's a VS2008 unit test that passes that illustrates this:

Class:

public class Foo
{
    public Foo(T val)
    {
        this.Value = val.ToString();
    }
    public Foo(string val)
    {
        this.Value = "--" + val + "--";
    }

    public string Value { get; set; }
}

Test method:

Foo f = new Foo("hello");
Assert.AreEqual("--hello--", f.Value);

Foo g = new Foo(10);
Assert.AreEqual("10", g.Value);

Type t = typeof(Foo);
t = t.GetGenericTypeDefinition();

Assert.AreEqual(2, t.GetConstructors().Length);

System.Reflection.ConstructorInfo c = t.GetConstructors()[0];
System.Reflection.ParameterInfo[] parms = c.GetParameters();
Assert.AreEqual(1, parms.Length);
Assert.IsTrue(parms[0].ParameterType.IsGenericParameter);

c = t.GetConstructors()[1];
parms = c.GetParameters();
Assert.AreEqual(1, parms.Length);
Assert.IsFalse(parms[0].ParameterType.IsGenericParameter);

The notable point here is the parms[0].ParameterType.IsGenericParameter check which checks if the parameter is a generic or not.

Once you've found your constructor then you've got the ConstructorInfo to pass to Emit.

public System.Reflection.ConstructorInfo FindStringConstructor(Type t)
{
    Type t2 = t.GetGenericTypeDefinition();

    System.Reflection.ConstructorInfo[] cs = t2.GetConstructors();
    for (int i = 0; i < cs.Length; i++)
    {
        if (cs[i].GetParameters()[0].ParameterType == typeof(string))
        {
            return t.GetConstructors()[i];
        }
    }

    return null;
}

Not exactly sure what your intention is though.