Add factory method.

MapTo/Extensions/StringBuilderExtensions.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Text;
+
+namespace MapTo.Extensions
+{
+    internal static class StringBuilderExtensions
+    {
+        public static StringBuilder PadLeft(this StringBuilder builder, int width)
+        {
+            for (var i = 0; i < width; i++)
+            {
+                builder.Append(" ");
+            }
+
+            return builder;
+        }
+
+        internal static StringBuilder AppendOpeningBracket(this StringBuilder builder, int indent = 0)
+        {
+            return builder.AppendLine().PadLeft(indent).AppendFormat("{{{0}", Environment.NewLine);
+        }
+
+        internal static StringBuilder AppendClosingBracket(this StringBuilder builder, int indent = 0, bool padNewLine = true)
+        {
+            if (padNewLine)
+            {
+                builder.AppendLine();
+            }
+
+            return builder.PadLeft(indent).Append("}");
+        }
+    }
+}

MapTo/SourceProvider.cs

@@ -45,9 +45,12 @@ namespace MapTo
         {
             var builder = new StringBuilder();
 
-            // Namespace declaration
             builder
                 .AppendFileHeader()
+                .GenerateUsings();
+
+            // Namespace declaration
+            builder
                 .AppendFormat("namespace {0}", model.Namespace)
                 .AppendOpeningBracket();
 
@@ -57,8 +60,11 @@ namespace MapTo
                 .AppendFormat("{0} class {1}", model.ClassModifiers, model.ClassName)
                 .AppendOpeningBracket(Indent1);
 
-            // Constructor declaration
+            // Class body
-            builder.GenerateConstructor(model, out var mappedProperties);
+            builder
+                .GenerateConstructor(model, out var mappedProperties)
+                .AppendLine()
+                .GenerateFactoryMethod(model);
 
             // End class declaration
             builder.AppendClosingBracket(Indent1);
@@ -69,6 +75,13 @@ namespace MapTo
             return (builder.ToString(), $"{model.ClassName}.cs");
         }
 
+        private static StringBuilder GenerateUsings(this StringBuilder builder)
+        {
+            return builder
+                .AppendLine("using System;")
+                .AppendLine();
+        }
+        
         private static StringBuilder GenerateConstructor(this StringBuilder builder, MapModel model, out List<IPropertySymbol> mappedProperties)
         {
             var destinationClassParameterName = model.DestinationClassName.ToCamelCase();
@@ -76,7 +89,10 @@ namespace MapTo
             builder
                 .PadLeft(Indent2)
                 .AppendFormat("public {0}({1} {2})", model.ClassName, model.DestinationClassName, destinationClassParameterName)
-                .AppendOpeningBracket(Indent2);
+                .AppendOpeningBracket(Indent2)
+                .PadLeft(Indent3)
+                .AppendFormat("if ({0} == null) throw new ArgumentNullException(nameof({0}));", destinationClassParameterName)
+                .AppendLine();
 
             mappedProperties = new List<IPropertySymbol>();
             foreach (var propertySymbol in model.DestinationTypeProperties)
@@ -94,41 +110,24 @@ namespace MapTo
             return builder.AppendClosingBracket(Indent2, padNewLine: false);
         }
 
-        private static StringBuilder AppendOpeningBracket(this StringBuilder builder, int indent = 0)
+        private static StringBuilder GenerateFactoryMethod(this StringBuilder builder, MapModel model)
         {
-            return builder.AppendLine().PadLeft(indent).AppendFormat("{{{0}", Environment.NewLine);
+            var destinationClassParameterName = model.DestinationClassName.ToCamelCase();
-        }
 
-        private static StringBuilder AppendClosingBracket(this StringBuilder builder, int indent = 0, bool padNewLine = true)
+            return builder
-        {
+                .AppendLine()
-            if (padNewLine)
+                .PadLeft(Indent2)
-            {
+                .AppendFormat("public static {0} From({1} {2})", model.ClassName, model.DestinationClassName, destinationClassParameterName)
-                builder.AppendLine();
+                .AppendOpeningBracket(Indent2)
-            }
+                .PadLeft(Indent3)
-
+                .AppendFormat("return {0} == null ? null : new {1}({0});", destinationClassParameterName, model.ClassName )
-            return builder.PadLeft(indent).Append("}");
+                .AppendClosingBracket(Indent2);
-        }
-
-        private static StringBuilder PadLeft(this StringBuilder builder, int width)
-        {
-            for (var i = 0; i < width; i++)
-            {
-                builder.Append(" ");
-            }
-
-            return builder;
         }
 
         private static StringBuilder AppendFileHeader(this StringBuilder builder)
         {
             return builder
-                .AppendLine("// <auto-generated />")
+                .AppendLine("// <auto-generated />");
-                .AppendLine();
-        }
-
-        private static string GetValueText(this NameSyntax? nameSyntax)
-        {
-            return (nameSyntax as IdentifierNameSyntax)?.Identifier.ValueText ?? string.Empty;
         }
     }
 }

MapToTests/Tests.cs

@@ -84,6 +84,7 @@ namespace Test
             
             const string expectedResult = @"
 // <auto-generated />
+using System;
 
 namespace Test
 {
@@ -91,9 +92,8 @@ namespace Test
     {
         public Foo(Baz baz)
         {
+            if (baz == null) throw new ArgumentNullException(nameof(baz));
         }
-    }
-}
 ";
             
             // Act
@@ -102,7 +102,7 @@ namespace Test
             // Assert
             diagnostics.ShouldBeSuccessful();
             compilation.SyntaxTrees.Count().ShouldBe(3);
-            compilation.SyntaxTrees.Last().ToString().ShouldBe(expectedResult.Trim());
+            compilation.SyntaxTrees.Last().ToString().ShouldStartWith(expectedResult.Trim());
         }
         
         [Fact]
@@ -127,6 +127,7 @@ namespace Test
             
             const string expectedResult = @"
 // <auto-generated />
+using System;
 
 namespace Test
 {
@@ -134,9 +135,8 @@ namespace Test
     {
         public Foo(Baz baz)
         {
+            if (baz == null) throw new ArgumentNullException(nameof(baz));
         }
-    }
-}
 ";
             
             // Act
@@ -145,7 +145,7 @@ namespace Test
             // Assert
             diagnostics.ShouldBeSuccessful();
             compilation.SyntaxTrees.Count().ShouldBe(3);
-            compilation.SyntaxTrees.Last().ToString().ShouldBe(expectedResult.Trim());
+            compilation.SyntaxTrees.Last().ToString().ShouldStartWith(expectedResult.Trim());
         }
 
         [Fact]
@@ -159,6 +159,7 @@ namespace Test
     {
         public Foo(Baz baz)
         {
+            if (baz == null) throw new ArgumentNullException(nameof(baz));
             Prop1 = baz.Prop1;
             Prop2 = baz.Prop2;
             Prop3 = baz.Prop3;
@@ -173,6 +174,28 @@ namespace Test
             compilation.SyntaxTrees.Count().ShouldBe(3);
             compilation.SyntaxTrees.Last().ToString().ShouldContain(expectedResult.Trim());
         }
+        
+        [Fact]
+        public void When_SourceTypeHasMatchingProperties_Should_CreateFromStaticMethod()
+        {
+            // Arrange
+            var source = GetSourceText();
+            
+            const string expectedResult = @"
+        public static Foo From(Baz baz)
+        {
+            return baz == null ? null : new Foo(baz);
+        }
+";
+            
+            // Act
+            var (compilation, diagnostics) = CSharpGenerator.GetOutputCompilation(source);
+            
+            // Assert
+            diagnostics.ShouldBeSuccessful();
+            compilation.SyntaxTrees.Count().ShouldBe(3);
+            compilation.SyntaxTrees.Last().ToString().ShouldContain(expectedResult.Trim());
+        }
 
         private static string GetSourceText(bool includeAttributeNamespace = false)
         {