How Can I Reliably Determine if a Type is Nullable in C#?

An improved method to reliably determine whether a C# type is nullable

To determine whether an object is explicitly declared as a nullable type, you need to know its underlying type and check whether it is a nullable value type. While this can be achieved using value conversion and reflection, this approach is not comprehensive as it excludes nullable reference types.

A more robust solution is to use generics and type inference to determine the nullability of a given type, as follows:

<code class="language-csharp">static bool IsNullable<T>(T obj)
{
    if (obj == null) return true; // 明显可空
    Type type = typeof(T);
    if (!type.IsValueType) return true; // 引用类型
    if (Nullable.GetUnderlyingType(type) != null) return true; // Nullable<T>
    return false; // 值类型
}</code>

This method utilizes the Nullable.GetUnderlyingType method to determine whether the provided type is of Nullable<T> type. By determining the underlying type, it can accurately distinguish between nullable and non-nullable value types.

While this method is very efficient when working with generic types, it may be less efficient when working with boxed value types. If the value has been boxed into an object, its underlying type is more difficult to identify.

For more information on identifying nullable types, please refer to:

• Microsoft Documentation: How to identify nullable types

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