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
{
    public String? Bar { get; set; }
}

Is there a way to check a class property uses a nullable reference type via reflection?

Answer 1

In .NET 6, the NullabilityInfoContext APIs were added to handle this. See this answer.

---

Prior to this, you need to read the attributes yourself. This appears to work, at least on the types I've tested it with.

public static bool IsNullable(PropertyInfo property) =>
    IsNullableHelper(property.PropertyType, property.DeclaringType, property.CustomAttributes);

public static bool IsNullable(FieldInfo field) =>
    IsNullableHelper(field.FieldType, field.DeclaringType, field.CustomAttributes);

public static bool IsNullable(ParameterInfo parameter) =>
    IsNullableHelper(parameter.ParameterType, parameter.Member, parameter.CustomAttributes);

private static bool IsNullableHelper(Type memberType, MemberInfo? declaringType, IEnumerable<CustomAttributeData> customAttributes)
{
    if (memberType.IsValueType)
        return Nullable.GetUnderlyingType(memberType) != null;

    var nullable = 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;
        }
    }

    for (var type = declaringType; type != null; type = type.DeclaringType)
    {
        var context = type.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".

Answer 2

.NET 6 Preview 7 adds reflection APIs to get nullability info.

Libraries: Reflection APIs for nullability information

Obviously, this only helps folks targeting .NET 6+.

Getting top-level nullability information

Imagine you’re implementing a serializer. Using these new APIs the serializer can check whether a given property can be set to null:

private NullabilityInfoContext _nullabilityContext = new NullabilityInfoContext();

private void DeserializePropertyValue(PropertyInfo p, object instance, object? value)
{
    if (value is null)
    {
        var nullabilityInfo = _nullabilityContext.Create(p);
        if (nullabilityInfo.WriteState is not NullabilityState.Nullable)
        {
            throw new MySerializerException($"Property '{p.GetType().Name}.{p.Name}'' cannot be set to null.");
        }
    }

    p.SetValue(instance, value);
}

Answer 3

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 4

Late answer.

This is what I ended up using thanks to Bill Menees:

static bool IsMarkedAsNullable(PropertyInfo p)
{
    return new NullabilityInfoContext().Create(p).WriteState is NullabilityState.Nullable;
}

// Tests:

class Foo
{
        public int Int1 { get; set; }
        public int? Int2 { get; set; } = null;


        public string Str1 { get; set; } = "";
        public string? Str2 { get; set; } = null;

        
        public List<Foo> Lst1 { get; set; } = new();
        public List<Foo>? Lst2 { get; set; } = null;


        public Dictionary<int, object> Dic1 { get; set; } = new();
        public Dictionary<int, object>? Dic2 { get; set; } = null;
}

....

var props = typeof(Foo).GetProperties();
foreach(var prop in props)
{
    Console.WriteLine($"Prop:{prop.Name} IsNullable:{IsMarkedAsNullable(prop)}");
}


// outputs:

Prop:Int1 IsNullable:False
Prop:Int2 IsNullable:True
Prop:Str1 IsNullable:False
Prop:Str2 IsNullable:True
Prop:Lst1 IsNullable:False
Prop:Lst2 IsNullable:True
Prop:Dic1 IsNullable:False
Prop:Dic2 IsNullable:True

Answer 5

A great answer there by @rico-suter !

The following is for those who just want a quick cut-and-paste solution until the real McCoy is available (see the proposal https://github.com/dotnet/runtime/issues/29723 ).

I put this together based on @canton7's post above plus a short look at the ideas in @rico-suter's code. The change from the @canton7's code is just abstracting the list of attribute sources and including a few new ones.

    private static bool IsAttributedAsNonNullable(this PropertyInfo propertyInfo)
    {
        return IsAttributedAsNonNullable(
            new dynamic?[] { propertyInfo },
            new dynamic?[] { propertyInfo.DeclaringType, propertyInfo.DeclaringType?.DeclaringType, propertyInfo.DeclaringType?.GetTypeInfo() }
        );
    }

    private static bool IsAttributedAsNonNullable(this ParameterInfo parameterInfo)
    {
        return IsAttributedAsNonNullable(
            new dynamic?[] { parameterInfo },
            new dynamic?[] { parameterInfo.Member, parameterInfo.Member.DeclaringType, parameterInfo.Member.DeclaringType?.DeclaringType, parameterInfo.Member.DeclaringType?.GetTypeInfo()
        );
    }

    private static bool IsAttributedAsNonNullable( dynamic?[] nullableAttributeSources, dynamic?[] nullableContextAttributeSources)
    {
        foreach (dynamic? nullableAttributeSource in nullableAttributeSources) {
            if (nullableAttributeSource == null) { continue; }
            CustomAttributeData? nullableAttribute = ((IEnumerable<CustomAttributeData>)nullableAttributeSource.CustomAttributes).FirstOrDefault(x => x.AttributeType.FullName == "System.Runtime.CompilerServices.NullableAttribute");
            if (nullableAttribute != null && nullableAttribute.ConstructorArguments.Count == 1) {
                CustomAttributeTypedArgument attributeArgument = nullableAttribute.ConstructorArguments[0];
                if (attributeArgument.ArgumentType == typeof(byte[])) {
                    var args = (ReadOnlyCollection<CustomAttributeTypedArgument>)(attributeArgument.Value ?? throw new NullReferenceException("Logic error!"));
                    if (args.Count > 0 && args[0].ArgumentType == typeof(byte)) {
                        byte value = (byte)(args[0].Value ?? throw new NullabilityLogicException());
                        return value == 1; // 0 = oblivious, 1 = nonnullable, 2 = nullable
                    }
                } else if (attributeArgument.ArgumentType == typeof(byte)) {
                    byte value = (byte)(attributeArgument.Value ?? throw new NullReferenceException("Logic error!"));
                    return value == 1;  // 0 = oblivious, 1 = nonnullable, 2 = nullable
                } else {
                    throw new InvalidOperationException($"Unrecognized argument type for NullableAttribute.");
                }
            }
        }
        foreach (dynamic? nullableContextAttributeSource in nullableContextAttributeSources) {
            if (nullableContextAttributeSource == null) { continue; }
            CustomAttributeData? nullableContextAttribute = ((IEnumerable<CustomAttributeData>)nullableContextAttributeSource.CustomAttributes).FirstOrDefault(x => x.AttributeType.FullName == "System.Runtime.CompilerServices.NullableContextAttribute");
            if (nullableContextAttribute != null && nullableContextAttribute.ConstructorArguments.Count == 1) {
                CustomAttributeTypedArgument attributeArgument = nullableContextAttribute.ConstructorArguments[0];
                if (attributeArgument.ArgumentType == typeof(byte)) {
                    byte value = (byte)(nullableContextAttribute.ConstructorArguments[0].Value ?? throw new NullabilityLogicException());
                    return value == 1;
                } else {
                    throw new InvalidOperationException($"Unrecognized argument type for NullableContextAttribute.");
                }
            }
        }
        return false;
    }

Answer 6

It's only the string? which gets a bit tricky. The rest of the nullable types are pretty straightforward to find out. For strings I used the following method, which you need to pass in a PropertyInfo object taken via reflection.

private bool IsNullable(PropertyInfo prop)
{
  return prop.CustomAttributes.Any(x => x.AttributeType.Name == "NullableAttribute");
}