How to Determine Nullable Reference Types Using .NET Reflection?

Use .NET reflection to check nullable reference types

.NET 6 introduced the NullabilityInfoContext API specifically to handle this task.

Pre-.NET 6 solutions

Before the NullabilityInfoContext API, you could manually inspect properties to determine nullability:

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)
{
    // 检查属性本身的 [Nullable] 属性。
    var nullable = customAttributes.FirstOrDefault(x => x.AttributeType.FullName == "System.Runtime.CompilerServices.NullableAttribute");
    if (nullable != null && nullable.ConstructorArguments.Count == 1)
    {
        // 检查第一个参数的值（表示可空性上下文的字节）。
        var args = (ReadOnlyCollection<CustomAttributeTypedArgument>)nullable.ConstructorArguments[0].Value!;
        if (args.Count > 0 && args[0].ArgumentType == typeof(byte))
        {
            return (byte)args[0].Value! == 2; // 2 代表“可空”。
        }
    }

    // 检查封闭类型的 [NullableContext] 属性。
    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))
        {
            // 检查第一个参数的值（表示可空性上下文的字节）。
            return (byte)context.ConstructorArguments[0].Value! == 2;
        }
    }

    // 未找到合适的属性，因此返回 false。
    return false;
}

This method examines the [Nullable] and [NullableContext] attributes and considers their various forms and meanings. However, it does not handle the case where properties are embedded in an assembly, which would require reflection-specific loading of types from different assemblies.

Other notes

• Generic types: Generic types may have [Nullable] properties instantiated with an array, where the first element represents the actual property and subsequent elements represent the generic parameters.
• Nullability context: A nullability context value of 2 means "nullable", 1 means "not nullable", and 0 means "ignore".

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