Set readonly property value using reflection C#

For some testing purpose, I need to set value for readonly property. Here is the code...

 public void SetPrivatePropertyValue<T>(object obj, string propName, T val)
        {
            Type t = obj.GetType();
            var prop = t.GetProperty(propName, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
            if (prop == null)
                throw new ArgumentOutOfRangeException("propName", string.Format("Property {0} was not found in Type {1}", propName, obj.GetType().FullName));
            t.InvokeMember(propName, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.SetProperty | BindingFlags.Instance, null, obj, new object[] { val });
        }

but that giving me error like

Method '<methodName>' not found

I also tried with

prop.SetValue(obj, true, null);

but error is:

Property set method not found

Can anyone tell me what's wrong here.