use NotNullWhen

Dapper/NRT.cs

@@ -0,0 +1,12 @@
+#if !NET5_0_OR_GREATER
+namespace System.Diagnostics.CodeAnalysis
+{
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class NotNullWhenAttribute : Attribute
+    {
+        public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        public bool ReturnValue { get; }
+    }
+}
+#endif

Dapper/SqlMapper.DapperRow.cs

@@ -46,14 +46,14 @@ internal bool TryGetValue(int index, out object? value)
             {
                 if (index < 0)
                 { // doesn't exist
-                    value = null!;
+                    value = null;
                     return false;
                 }
                 // exists, **even if** we don't have a value; consider table rows heterogeneous
                 value = index < values.Length ? values[index] : null;
                 if (value is DeadValue)
                 { // pretend it isn't here
-                    value = null!;
+                    value = null;
                     return false;
                 }
                 return true;

Dapper/SqlMapper.Link.cs

@@ -1,4 +1,5 @@
-using System.Threading;
+using System.Diagnostics.CodeAnalysis;
+using System.Threading;
 
 namespace Dapper
 {
@@ -14,18 +15,18 @@ public static partial class SqlMapper
         internal class Link<TKey, TValue> where TKey : class
         {
             public static void Clear(ref Link<TKey, TValue>? head) => Interlocked.Exchange(ref head, null);
-            public static bool TryGet(Link<TKey, TValue>? link, TKey key, out TValue value)
+            public static bool TryGet(Link<TKey, TValue>? link, TKey key, [NotNullWhen(true)] out TValue? value)
             {
                 while (link is not null)
                 {
                     if ((object)key == (object)link.Key)
                     {
-                        value = link.Value;
+                        value = link.Value!;
                         return true;
                     }
                     link = link.Tail;
                 }
-                value = default!;
+                value = default;
                 return false;
             }
 
@@ -35,7 +36,7 @@ public static bool TryAdd(ref Link<TKey, TValue>? head, TKey key, ref TValue val
                 do
                 {
                     var snapshot = Interlocked.CompareExchange(ref head, null, null);
-                    if (TryGet(snapshot, key, out TValue found))
+                    if (TryGet(snapshot, key, out TValue? found))
                     { // existing match; report the existing value instead
                         value = found;
                         return false;

Dapper/SqlMapper.cs

@@ -89,14 +89,15 @@ private static void CollectCacheGarbage()
 
         private const int COLLECT_PER_ITEMS = 1000, COLLECT_HIT_COUNT_MIN = 0;
         private static int collect;
-        private static bool TryGetQueryCache(Identity key, out CacheInfo value)
+
+        private static bool TryGetQueryCache(Identity key, [NotNullWhen(true)] out CacheInfo? value)
         {
             if (_queryCache.TryGetValue(key, out value!))
             {
                 value.RecordHit();
                 return true;
             }
-            value = null!;
+            value = null;
             return false;
         }
 
@@ -1817,7 +1818,7 @@ private static int GetNextSplit(int startIdx, string splitOn, DbDataReader reade
 
         private static CacheInfo GetCacheInfo(Identity identity, object? exampleParameters, bool addToCache)
         {
-            if (!TryGetQueryCache(identity, out CacheInfo info))
+            if (!TryGetQueryCache(identity, out CacheInfo? info))
             {
                 if (GetMultiExec(exampleParameters) is not null)
                 {