How do I use reflection to invoke a private method?

Microsoft recently modified the reflection API rendering most of these answers obsolete. The following should work on modern platforms (including Xamarin.Forms and UWP):

obj.GetType().GetTypeInfo().GetDeclaredMethod("MethodName").Invoke(obj, yourArgsHere);

Or as an extension method:

public static object InvokeMethod<T>(this T obj, string methodName, params object[] args)
{
    var type = typeof(T);
    var method = type.GetTypeInfo().GetDeclaredMethod(methodName);
    return method.Invoke(obj, args);
}

Note:

• If the desired method is in a superclass of obj the T generic must be explicitly set to the type of the superclass.

• If the method is asynchronous you can use await (Task) obj.InvokeMethod(…).

Simply change your code to use the overloaded version of GetMethod that accepts BindingFlags:

MethodInfo dynMethod = this.GetType().GetMethod("Draw_" + itemType, 
    BindingFlags.NonPublic | BindingFlags.Instance);
dynMethod.Invoke(this, new object[] { methodParams });

Here's the BindingFlags enumeration documentation.

And if you really want to get yourself in trouble, make it easier to execute by writing an extension method:

static class AccessExtensions
{
    public static object call(this object o, string methodName, params object[] args)
    {
        var mi = o.GetType ().GetMethod (methodName, System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance );
        if (mi != null) {
            return mi.Invoke (o, args);
        }
        return null;
    }
}

And usage:

    class Counter
    {
        public int count { get; private set; }
        void incr(int value) { count += value; }
    }

    [Test]
    public void making_questionable_life_choices()
    {
        Counter c = new Counter ();
        c.call ("incr", 2);             // "incr" is private !
        c.call ("incr", 3);
        Assert.AreEqual (5, c.count);
    }

BindingFlags.NonPublic will not return any results by itself. As it turns out, combining it with BindingFlags.Instance does the trick.

MethodInfo dynMethod = this.GetType().GetMethod("Draw_" + itemType, 
    BindingFlags.NonPublic | BindingFlags.Instance);