Add Assumes.NotNull

Sometimes the compiler cannot know that a reference is not null. Using this method instructs it, and validates the fact at runtime in debug builds.

Author: drewnoakes

src/NetMQ/Utils/Assumes.cs

@@ -0,0 +1,18 @@
+#nullable enable
+
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+
+namespace NetMQ
+{
+    internal static class Assumes
+    {
+#pragma warning disable CS8777 // Parameter must have a non-null value when exiting.
+        [Conditional("DEBUG")]
+        public static void NotNull<T>([NotNull] T o) where T : class?
+        {
+            Debug.Assert(o is object);
+        }
+#pragma warning restore CS8777 // Parameter must have a non-null value when exiting.
+    }
+}