c#, is nullable, IsNullable, How to check if an object is nullable?

IsNullable.cs

///
/// How to check if an object is nullable?
/// ref →　http://stackoverflow.com/questions/374651/how-to-check-if-an-object-is-nullable
///

static bool IsNullable<T>(T obj)
{
    if (obj == null) return true; // obvious
    Type type = typeof(T);
    if (!type.IsValueType) return true; // ref-type
    if (Nullable.GetUnderlyingType(type) != null) return true; // Nullable<T>
    return false; // value-type
}

static bool IsNullable<T>()
{
    Type type = typeof(T);
    if (!type.IsValueType) return true; // ref-type
    if (Nullable.GetUnderlyingType(type) != null) return true; // Nullable<T>
    return false; // value-type
}

//# Testing
Decimal C = 9;
Decimal? D = 9;
bool b1 = IsNullable(C); // false
bool b2 = IsNullable(D); // ture
bool b3 = IsNullable<Decimal>(); // false
bool b4 = IsNullable<Decimal?>(); // ture

Gist:

https://sharplab.io/#v2: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=