Support caching root-level parameter expressions.

Changelog.txt

@@ -1,6 +1,8 @@
 11.9.0 -
 Fix memory leak in NotEmptyValidator/EmptyValidator (#2174)
 Add more descriptive error messages if a rule throws a NullReferenceException (#2152)
+Add support for caching root parameter expressions (eg RuleFor(x => x)) (#2168)
+Add builds for .net 8
 
 11.8.1 - 22 Nov 2023
 Fix unintentional behavioural changes in introduced in the previous release as part of #2158

src/FluentValidation.Tests/AccessorCacheTests.cs

@@ -50,6 +50,32 @@ public class AccessorCacheTests {
 		Assert.Equal(compiled3, compiled4);
 	}
 
+	[Fact]
+	public void Gets_accessor_for_parameter_expression() {
+		Expression<Func<Person, Person>> expr1 = x => x;
+
+		var compiled1 = expr1.Compile();
+		var compiled2 = expr1.Compile();
+
+		Assert.NotEqual(compiled1, compiled2);
+
+		var compiled3 = AccessorCache<Person>.GetCachedAccessor(null, expr1);
+		var compiled4 = AccessorCache<Person>.GetCachedAccessor(null, expr1);
+
+		// Expression should have been cached.
+		Assert.Equal(compiled3, compiled4);
+
+		Expression<Func<Person, Person>> expr2 = x => x;
+		var compiled5 = AccessorCache<Person>.GetCachedAccessor(null, expr2);
+
+		// Technically a different expression, but for the same type, so should still be cached.
+		Assert.Equal(compiled3, compiled5);
+
+		// Different expression for a different type. Shouldn't try and cache and cast.
+		Expression<Func<Address, Address>> expr3 = x => x;
+		var compiled6 = AccessorCache<Address>.GetCachedAccessor(null, expr3);
+	}
+
 	[Fact]
 	public void Equality_comparison_check() {
 		Expression<Func<Person, string>> expr1 = x => x.Surname;

src/FluentValidation/Internal/AccessorCache.cs

@@ -10,7 +10,8 @@ namespace FluentValidation.Internal;
 /// </summary>
 /// <typeparam name="T"></typeparam>
 public static class AccessorCache<T> {
-	private static readonly ConcurrentDictionary<Key, Delegate> _cache = new ConcurrentDictionary<Key, Delegate>();
+	private static readonly ConcurrentDictionary<Key, Delegate> _cache = new();
+	private static readonly ConcurrentDictionary<Type, Delegate> _parameterAccessorCache = new();
 
 	/// <summary>
 	/// Gets an accessor func based on an expression
@@ -22,11 +23,27 @@ public static class AccessorCache<T> {
 	/// <param name="cachePrefix">Cache prefix</param>
 	/// <returns>Accessor func</returns>
 	public static Func<T, TProperty> GetCachedAccessor<TProperty>(MemberInfo member, Expression<Func<T, TProperty>> expression, bool bypassCache = false, string cachePrefix = null) {
-		if (member == null || bypassCache || ValidatorOptions.Global.DisableAccessorCache) {
+		if (bypassCache || ValidatorOptions.Global.DisableAccessorCache) {
 			return expression.Compile();
 		}
 
-		var key = new Key(member, expression, cachePrefix);
+		Key key;
+
+		if (member == null) {
+			// If the expression doesn't reference a property we don't support
+			// caching it, The only exception is parameter expressions referencing the same object (eg RuleFor(x => x))
+			if (expression.IsParameterExpression() && typeof(T) == typeof(TProperty)) {
+				key = new Key(null, expression, typeof(T).FullName + ":" + cachePrefix);
+			}
+			else {
+				// Unsupported expression type. Non cacheable.
+				return expression.Compile();
+			}
+		}
+		else {
+			key = new Key(member, expression, cachePrefix);
+		}
+
 		return (Func<T,TProperty>)_cache.GetOrAdd(key, k => expression.Compile());
 	}