System.Diagnostics.CodeAnalysis

 namespace System.Diagnostics.CodeAnalysis {
+    public sealed class AllowNullAttribute : Attribute {
+        public AllowNullAttribute();
+    }
+    public sealed class DisallowNullAttribute : Attribute {
+        public DisallowNullAttribute();
+    }
+    public sealed class DoesNotReturnAttribute : Attribute {
+        public DoesNotReturnAttribute();
+    }
+    public sealed class DoesNotReturnIfAttribute : Attribute {
+        public DoesNotReturnIfAttribute(bool parameterValue);
+        public bool ParameterValue { get; }
+    }
+    public sealed class MaybeNullAttribute : Attribute {
+        public MaybeNullAttribute();
+    }
+    public sealed class MaybeNullWhenAttribute : Attribute {
+        public MaybeNullWhenAttribute(bool returnValue);
+        public bool ReturnValue { get; }
+    }
+    public sealed class NotNullAttribute : Attribute {
+        public NotNullAttribute();
+    }
+    public sealed class NotNullIfNotNullAttribute : Attribute {
+        public NotNullIfNotNullAttribute(string parameterName);
+        public string ParameterName { get; }
+    }
+    public sealed class NotNullWhenAttribute : Attribute {
+        public NotNullWhenAttribute(bool returnValue);
+        public bool ReturnValue { get; }
+    }
 }