NullableAttributes.cs

namespace System.Diagnostics.CodeAnalysis {

    // Input:  field, property, argument
    // Output: field, property, argument, result

    // AllowNull            - Null input is allowed
    // DisallowNull         - Null input is disallowed

    // MaybeNull            - Output may be null
    // MaybeNull/When       - Output may be null (when result == true/false)

    // NotNull              - Output will be non-null
    // NotNull/When         - Output will be non-null (when result   == true/false)
    // NotNull/If/NotNull   - Output will be non-null (if   argument == non-null)

    // MemberNotNull        - Method/property will ensure that the field/property is non-null
    // MemberNotNull/When   - Method/property will ensure that the field/property is non-null (when result == true/false)

    // DoesNotReturn        - Method will never return
    // DoesNotReturn/If     - Method will never return (if argument == true/false)


    // Input
    [AttributeUsage( AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false )]
    public sealed class AllowNullAttribute : Attribute {
    }
    [AttributeUsage( AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false )]
    public sealed class DisallowNullAttribute : Attribute {
    }


    // Output/MaybeNull
    [AttributeUsage( AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false )]
    public sealed class MaybeNullAttribute : Attribute {
    }
    [AttributeUsage( AttributeTargets.Parameter, Inherited = false )]
    public sealed class MaybeNullWhenAttribute : Attribute {
        public bool ReturnValue { get; }
        public MaybeNullWhenAttribute(bool returnValue) {
            ReturnValue = returnValue;
        }
    }


    // Output/NotNull
    [AttributeUsage( AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false )]
    public sealed class NotNullAttribute : Attribute {
    }
    [AttributeUsage( AttributeTargets.Parameter, Inherited = false )]
    public sealed class NotNullWhenAttribute : Attribute {
        public bool ReturnValue { get; }
        public NotNullWhenAttribute(bool returnValue) {
            ReturnValue = returnValue;
        }
    }
    [AttributeUsage( AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false )]
    public sealed class NotNullIfNotNullAttribute : Attribute {
        public string ParameterName { get; }
        public NotNullIfNotNullAttribute(string parameterName) {
            ParameterName = parameterName;
        }
    }


    // Ensure
    [AttributeUsage( AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true )]
    public sealed class MemberNotNullAttribute : Attribute {
        public string[] Members { get; }
        public MemberNotNullAttribute(string member) {
            Members = new[] { member };
        }
        public MemberNotNullAttribute(params string[] members) {
            Members = members;
        }
    }
    [AttributeUsage( AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true )]
    public sealed class MemberNotNullWhenAttribute : Attribute {
        public bool ReturnValue { get; }
        public string[] Members { get; }
        public MemberNotNullWhenAttribute(bool returnValue, string member) {
            ReturnValue = returnValue;
            Members = new[] { member };
        }
        public MemberNotNullWhenAttribute(bool returnValue, params string[] members) {
            ReturnValue = returnValue;
            Members = members;
        }
    }


    // DoesNotReturn
    [AttributeUsage( AttributeTargets.Method, Inherited = false )]
    public sealed class DoesNotReturnAttribute : Attribute {
    }
    [AttributeUsage( AttributeTargets.Parameter, Inherited = false )]
    public sealed class DoesNotReturnIfAttribute : Attribute {
        public bool ParameterValue { get; }
        public DoesNotReturnIfAttribute(bool parameterValue) {
            ParameterValue = parameterValue;
        }
    }


}