Why does a dynamic parameter in a generic method throw a null reference exception when using an Object? [duplicate]

Question

I wonder if someone could explain why in this code

public class SomeClass
{
    public T GenericMethod<T>(dynamic value)
    {
        return (T)value;
    }
}

the 'return value;' statement throws an null reference exception when called with:

new SomeClass().GenericMethod<object>(new object()); // throws System.NullReferenceException

It works as expected when called with:

new SomeClass().GenericMethod<string>("SomeString"); // returns SomeString
new SomeClass().GenericMethod<object>("SomeString"); // returns SomeString

Note: The following compiles and runs just fine

public class SomeOtherClass
{
    public T GenericMethod<T>(object value)
    {
        return (T)value;
    }
}

The stacktrace:

System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.CSharp.RuntimeBinder.ExpressionTreeCallRewriter.GenerateLambda(EXPRCALL pExpr)
   at Microsoft.CSharp.RuntimeBinder.Semantics.ExprVisitorBase.Visit(EXPR pExpr)
   at Microsoft.CSharp.RuntimeBinder.ExpressionTreeCallRewriter.Rewrite(TypeManager typeManager, EXPR pExpr, IEnumerable`1 listOfParameters)
   at Microsoft.CSharp.RuntimeBinder.RuntimeBinder.BindCore(DynamicMetaObjectBinder payload, IEnumerable`1 parameters, DynamicMetaObject[] args, DynamicMetaObject& deferredBinding)
   at Microsoft.CSharp.RuntimeBinder.RuntimeBinder.Bind(DynamicMetaObjectBinder payload, IEnumerable`1 parameters, DynamicMetaObject[] args, DynamicMetaObject& deferredBinding)
   at Microsoft.CSharp.RuntimeBinder.BinderHelper.Bind(DynamicMetaObjectBinder action, RuntimeBinder binder, IEnumerable`1 args, IEnumerable`1 arginfos, DynamicMetaObject onBindingError)
   at Microsoft.CSharp.RuntimeBinder.CSharpConvertBinder.FallbackConvert(DynamicMetaObject target, DynamicMetaObject errorSuggestion)
   at System.Dynamic.DynamicMetaObject.BindConvert(ConvertBinder binder)
   at System.Dynamic.DynamicMetaObjectBinder.Bind(Object[] args, ReadOnlyCollection`1 parameters, LabelTarget returnLabel)
   at System.Runtime.CompilerServices.CallSiteBinder.BindCore[T](CallSite`1 site, Object[] args)
   at System.Dynamic.UpdateDelegates.UpdateAndExecute1[T0,TRet](CallSite site, T0 arg0)

Answer 1

It appears it's a bug first reported in 2012 but still apparently not fixed (as of May 24th 2016)

Answer 2

Update

As the marked duplicate indicates this is a known bug in the .NET runtime.

---

The problem is likely a poor indication of an invalid run-time cast. If you take out the dynamic binding and make the parameter type object you get the compiler error

Cannot implicitly convert type 'object' to 'T'. An explicit conversion exists (are you missing a cast?)

Since by adding dynamic this casting is done at run-time, this implicit conversion error manifests itself differently at run-time with a vague NullReferenceException in the run-time bindings.

I'm not an expert in the inner workings of the DLR, but I suspect that an object passed as a dynamic value is not really a pure object at run-time. I suspect that it's some sort of wrapper around object, and thus can't be implicitly cast to object at run-time.

An explicit reference cast

return (T)(object)value;

will not generate that error.