How to use .NET reflection to check for nullable reference type

Question

C# 8.0 introduces nullable reference types. Here\'s a simple class with a nullable property:

public class Foo
{
    pu         


        

Answer 1

I wrote a library to do reflection of NRT types - internally it looks at the generated attributes and gives you a simple API:

https://github.com/RicoSuter/Namotion.Reflection

Answer 2

This appears to work, at least on the types I've tested it with.

You need to pass the PropertyInfo for the property you're interested in.

public static bool IsNullable(PropertyInfo property)
{
    if (property.PropertyType.IsValueType)
        return Nullable.GetUnderlyingType(property.PropertyType) != null;
    
    var nullable = property.CustomAttributes
        .FirstOrDefault(x => x.AttributeType.FullName == "System.Runtime.CompilerServices.NullableAttribute");
    if (nullable != null && nullable.ConstructorArguments.Count == 1)
    {
        var attributeArgument = nullable.ConstructorArguments[0];
        if (attributeArgument.ArgumentType == typeof(byte[]))
        {
            var args = (ReadOnlyCollection<CustomAttributeTypedArgument>)attributeArgument.Value;
            if (args.Count > 0 && args[0].ArgumentType == typeof(byte))
            {
                return (byte)args[0].Value == 2;
            }
        }
        else if (attributeArgument.ArgumentType == typeof(byte))
        {
            return (byte)attributeArgument.Value == 2;
        }
    }

    var context = property.DeclaringType.CustomAttributes
        .FirstOrDefault(x => x.AttributeType.FullName == "System.Runtime.CompilerServices.NullableContextAttribute");
    if (context != null &&
        context.ConstructorArguments.Count == 1 &&
        context.ConstructorArguments[0].ArgumentType == typeof(byte))
    {
        return (byte)context.ConstructorArguments[0].Value == 2;
    }

    // Couldn't find a suitable attribute
    return false;
}

See this document for details.

The general gist is that either the property itself can have a [Nullable] attribute on it, or if it doesn't the enclosing type might have [NullableContext] attribute. We first look for [Nullable], then if we don't find it we look for [NullableContext] on the enclosing type.

The compiler might embed the attributes into the assembly, and since we might be looking at a type from a different assembly, we need to do a reflection-only load.

[Nullable] might be instantiated with an array, if the property is generic. In this case, the first element represents the actual property (and further elements represent generic arguments). [NullableContext] is always instantiated with a single byte.

A value of 2 means "nullable". 1 means "not nullable", and 0 means "oblivious".