Use ConcurrentDictionary's GetOrAdd with delegate instead.

src/Microsoft.IdentityModel.Tokens/Validation/Results/Details/ValidationError.cs

@@ -288,16 +288,9 @@ public static StackFrame GetCurrentStackFrame(
             // String is allocated, but it goes out of scope immediately after the call
             string key = filePath + lineNumber;
 
-            // Try to get the stack frame from the cache,
-            // otherwise create a new one and add it to the cache
-            if (!CachedStackFrames.TryGetValue(key, out StackFrame? frame))
-            {
-                frame = new StackFrame(skipFrames, true);
-                CachedStackFrames.TryAdd(key, frame);
-            }
-
-            // By this point the frame cannot be null, but the compiler doesn't know that
-            return frame!;
+            return CachedStackFrames.GetOrAdd(
+                key,
+                _ => new StackFrame(skipFrames, true));
         }
 
         // ConcurrentDictionary is thread-safe and only locks when adding a new item.