Performance improvements for expression transformations

Author: DynamicField

NotSoAutoMapper/ExpressionProcessing/MapperInliningOperations.cs

@@ -29,7 +29,7 @@ public static Expression InlineCollection(
             // -----
             //  collection.Select(y => new Thing { Name = y.Name })
 
-            var mapper = mapperExpression.CompileAndGet<IMapper>() ?? throw TransformerExceptions.NullMapperException;
+            var mapper = mapperExpression.GetValue<IMapper>() ?? throw TransformerExceptions.NullMapperException;
 
             // The item type of the source : IEnumerable<int> -> int
             var sourceItemType = FindEnumerableType(sourceExpression.Type).GenericTypeArguments[0];
@@ -66,7 +66,7 @@ static LambdaExpression CreateSelectLambda(Type enumerableItemType, IMapper mapp
 
         public static Expression InlineObject(Expression sourceExpression, Expression mapperExpression)
         {
-            var mapper = mapperExpression.CompileAndGet<IMapper>();
+            var mapper = mapperExpression.GetValue<IMapper>();
             if (mapper is null)
             {
                 throw TransformerExceptions.NullMapperException;

NotSoAutoMapper/ExpressionProcessing/RetrieveExpressionValueExtensions.cs

@@ -1,13 +1,35 @@
 using System;
 using System.Linq.Expressions;
+using System.Reflection;
 
 namespace NotSoAutoMapper.ExpressionProcessing
 {
     internal static class RetrieveExpressionValueExtensions
     {
-        public static T? CompileAndGet<T>(this Expression expression)
+        public static T? GetValue<T>(this Expression expression)
         {
-            var func = (Func<T?>) Expression.Lambda(expression).Compile();
+            // Maintaining an expression cache is tricky, the Expression class doesn't implement .Equals!
+            // Instead, we have some well-known common cases to boost the performance.
+            // Reflection is much more faster than Expression compilation.
+            
+            if (expression is ConstantExpression constantExpression)
+            {
+                return (T?) constantExpression.Value;
+            }
+
+            if (expression is MemberExpression memberExpression)
+            {
+                var member = memberExpression.Member;
+                switch (member)
+                {
+                    case PropertyInfo propertyInfo:
+                        return (T?) propertyInfo.GetValue(memberExpression.Expression);
+                    case FieldInfo fieldInfo:
+                        return (T?) fieldInfo.GetValue(memberExpression.Expression);
+                }
+            }
+            
+            var func = (Func<T?>) Expression.Lambda(expression).Compile(true);
             return func();
         }
     }

NotSoAutoMapper/NotSoAutoMapper.csproj

@@ -7,7 +7,7 @@
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <Authors>jeuxjeux20</Authors>
     <Product />
-    <Description>A simple and extensible library to map DTOs (or anything!) using powerful expressions.</Description>
+    <Description>A library to map your DTOs with reusable expressions.</Description>
     <PackageProjectUrl>https://github.com/jeuxjeux20/NotSoAutoMapper</PackageProjectUrl>
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
     <PackageIcon>logo_512x512.png</PackageIcon>
@@ -16,6 +16,12 @@
     <RepositoryType>git</RepositoryType>
     <PackageTags>handmademapper mapper dto expression entityframework ef efcore entityframeworkcore map</PackageTags>
     <PackageReleaseNotes>
+      3.0.0-preview4:
+      * Creating a mapper is now much faster.
+      
+      3.0.0-preview3:
+      * Null bug fixes
+      
       3.0.0-preview2:
       * Added custom TransformedUsing attributes
       * Added inheritance for Merge
@@ -27,7 +33,7 @@
       * Removed recursion prevention as it is not longer possible to have recursion with eager-loading mappers
       * Removed MapperOptions
     </PackageReleaseNotes>
-    <Version>3.0.0-preview3</Version>
+    <Version>3.0.0-preview4</Version>
     <AssemblyVersion>3.0.0</AssemblyVersion>
     <FileVersion>3.0.0</FileVersion>
   </PropertyGroup>