Fixed #510 where existing structs were incorrectly mapped.

src/Mapster.Tests/WhenMappingStructRegression.cs

@@ -0,0 +1,80 @@
+using Mapster;
+using MapsterMapper;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Shouldly;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using static Mapster.Tests.WhenMappingStructRegression;
+
+namespace Mapster.Tests
+{
+    /// <summary>
+    /// Regression tests for https://github.com/MapsterMapper/Mapster/issues/510
+    /// </summary>
+    [TestClass]
+    public class WhenMappingStructRegression
+    {
+        public struct SourceClass
+        {
+            public string Name { get; set; }
+        }
+
+        public struct SourceStruct
+        {
+            public string Name { get; set; }
+        }
+        public struct DestinationStruct
+        {
+            public string Name { get; set; }
+
+            public string Ignore { get; set; }
+        }
+
+        [TestMethod]
+        public void TestMapStructToExistingStruct()
+        {
+
+            TypeAdapterConfig<SourceStruct, DestinationStruct>
+                .ForType()
+                .Ignore(s => s.Ignore);
+
+            var source = new SourceStruct
+            {
+                Name = "Some Name",
+            };
+            var dest = new DestinationStruct
+            {
+                Ignore = "Ignored property",
+            };
+            dest = source.Adapt(dest);
+
+            dest.Ignore.ShouldBe("Ignored property");
+            dest.Name.ShouldBe("Some Name");
+        }
+
+        [TestMethod]
+        public void TestMapClassToExistingStruct()
+        {
+
+            TypeAdapterConfig<SourceClass, DestinationStruct>
+                .ForType()
+                .Ignore(s => s.Ignore);
+
+            var source = new SourceClass
+            {
+                Name = "Some Name",
+            };
+            var dest = new DestinationStruct
+            {
+                Ignore = "Ignored property",
+            };
+            dest = source.Adapt(dest);
+
+            dest.Ignore.ShouldBe("Ignored property");
+            dest.Name.ShouldBe("Some Name");
+        }
+    }
+}

src/Mapster/Adapters/BaseAdapter.cs

@@ -196,10 +196,19 @@ protected Expression CreateBlockExpressionBody(Expression source, Expression? de
                 transformedSource = src;
             }
             var set = CreateInstantiationExpression(transformedSource, destination, arg);
-            if (destination != null && (this.UseTargetValue || arg.UseDestinationValue) && arg.GetConstructUsing()?.Parameters.Count != 2 && destination.CanBeNull())
+            if (destination != null && (this.UseTargetValue || arg.UseDestinationValue) && arg.GetConstructUsing()?.Parameters.Count != 2)
             {
-                //dest ?? new TDest();
-                set = Expression.Coalesce(destination, set);
+                if (destination.CanBeNull())
+                {
+                    //dest ?? new TDest();
+                    set = Expression.Coalesce(destination, set);
+                }
+                else if (destination.Type.IsValueType && !destination.CanBeNull())
+                {
+                    // Destination already exists, and this is a struct, so simply use the destination object
+                    set = destination;
+                }
+
             }
 
             if (set.NodeType == ExpressionType.Throw)