Separate the null check from non-null mapping

Author: Steven van Dam

src/AutoMapper.Extensions.ExpressionMapping/XpressionMapperVisitor.cs

@@ -102,7 +102,7 @@ Expression GetMappedMemberExpression(Expression parentExpression, List<PropertyM
             }
         }
 
-        protected MemberExpression GetMemberExpressionFromMemberMaps(string fullName, Expression visitedParentExpr)
+        protected MemberExpression GetMemberExpressionFromMemberMaps(string fullName, Expression visitedParentExpr) 
             => ExpressionHelpers.MemberAccesses(fullName, visitedParentExpr);
 
         private Expression GetMemberExpressionFromCustomExpression(PropertyMapInfo lastWithCustExpression,
@@ -130,7 +130,7 @@ Expression PrependParentMemberExpression(PrependParentNameVisitor visitor)
                 );
         }
 
-        protected Expression GetMemberExpressionFromCustomExpression(List<PropertyMapInfo> propertyMapInfoList, PropertyMapInfo lastWithCustExpression, Expression mappedParentExpr)
+        protected Expression GetMemberExpressionFromCustomExpression(List<PropertyMapInfo> propertyMapInfoList, PropertyMapInfo lastWithCustExpression, Expression mappedParentExpr) 
             => GetMemberExpressionFromCustomExpression
             (
                 lastWithCustExpression,
@@ -305,14 +305,16 @@ Expression DoVisitUnary(Expression updated)
 
         protected override Expression VisitConstant(ConstantExpression node)
         {
-            if (this.TypeMappings.TryGetValue(node.Type, out Type newType)
-                && ConfigurationProvider.ResolveTypeMap(newType, node.Type) != null)
-                //The destination becomes the source because to map a source expression to a destination expression,
-                //we need the expressions used to create the source from the destination
-                return base.VisitConstant(Expression.Constant(Mapper.MapObject(node.Value, node.Type, newType), newType));
-            //Issue 3455 (Non-Generic Mapper.Map failing for structs in v10)
-            //return base.VisitConstant(Expression.Constant(Mapper.Map(node.Value, node.Type, newType), newType));
+            if (this.TypeMappings.TryGetValue(node.Type, out Type newType))
+            {
+                if (node.Value == null)
+                    return base.VisitConstant(Expression.Constant(null, newType));
 
+                if (ConfigurationProvider.ResolveTypeMap(node.Type, newType) != null)
+                    return base.VisitConstant(Expression.Constant(Mapper.MapObject(node.Value, node.Type, newType), newType));
+                //Issue 3455 (Non-Generic Mapper.Map failing for structs in v10)
+                //return base.VisitConstant(Expression.Constant(Mapper.Map(node.Value, node.Type, newType), newType));
+            }
             return base.VisitConstant(node);
         }
 
@@ -412,13 +414,13 @@ private static void AddPropertyMapInfo(Type parentType, string name, List<Proper
             }
         }
 
-        private bool GenericTypeDefinitionsAreEquivalent(Type typeSource, Type typeDestination)
+        private bool GenericTypeDefinitionsAreEquivalent(Type typeSource, Type typeDestination) 
             => typeSource.IsGenericType() && typeDestination.IsGenericType() && typeSource.GetGenericTypeDefinition() == typeDestination.GetGenericTypeDefinition();
 
         protected void FindDestinationFullName(Type typeSource, Type typeDestination, string sourceFullName, List<PropertyMapInfo> propertyMapInfoList)
         {
             const string period = ".";
-
+            
             if (typeSource == typeDestination)
             {
                 var sourceFullNameArray = sourceFullName.Split(new[] { period[0] }, StringSplitOptions.RemoveEmptyEntries);
@@ -461,7 +463,7 @@ protected void FindDestinationFullName(Type typeSource, Type typeDestination, st
 
                     var sourceType = typeSource.GetFieldOrProperty(propertyName).GetMemberType();
                     var destType = typeDestination.GetFieldOrProperty(propertyName).GetMemberType();
-
+                    
                     TypeMappings.AddTypeMapping(ConfigurationProvider, sourceType, destType);
 
                     var childFullName = sourceFullName.Substring(sourceFullName.IndexOf(period, StringComparison.OrdinalIgnoreCase) + 1);