Add implicit operator to Union types (#438)

src/FlatSharp.Compiler/SchemaModel/ReferenceUnionSchemaModel.cs

@@ -102,6 +102,7 @@ protected override void OnWriteCode(CodeWriter writer, CompileContext context)
             {
                 this.WriteConstructor(writer, item.resolvedType, item.value, item.propertyType);
                 this.AddUnionMember(writer, item.resolvedType, item.value, item.propertyType, context);
+                this.WriteImplicitOperator(writer, item.resolvedType);
             }
 
             this.WriteDefaultConstructor(writer);
@@ -251,7 +252,16 @@ private void WriteConstructor(CodeWriter writer, string resolvedType, EnumVal un
         }
     }
 
-    private void WriteDefaultConstructor(CodeWriter writer)
+    private void WriteImplicitOperator(CodeWriter writer, string resolvedType)
+    {
+        writer.AppendLine($"public static implicit operator {this.Name}({resolvedType} value)");
+        using (writer.WithBlock())
+        {
+            writer.AppendLine($"return new {this.Name}(value);");
+        }
+    }
+
+        private void WriteDefaultConstructor(CodeWriter writer)
     {
         writer.AppendLine($"protected {this.Name}()");
         using (writer.WithBlock())

src/FlatSharp.Compiler/SchemaModel/ValueUnionSchemaModel.cs

@@ -175,6 +175,7 @@ protected override void OnWriteCode(CodeWriter writer, CompileContext context)
                 }
 
                 this.WriteConstructor(writer, item.resolvedType, item.value, propertyClrType, generateUnsafeItems);
+                this.WriteImplicitOperator(writer, item.resolvedType);
                 this.WriteUncheckedGetItemMethod(writer, item.resolvedType, item.value, propertyClrType, generateUnsafeItems);
 
                 writer.AppendLine();
@@ -363,4 +364,13 @@ private void WriteConstructor(CodeWriter writer, string resolvedType, EnumVal un
             }
         }
     }
+
+    private void WriteImplicitOperator(CodeWriter writer, string resolvedType)
+    {
+        writer.AppendLine($"public static implicit operator {this.Name}({resolvedType} value)");
+        using (writer.WithBlock())
+        {
+            writer.AppendLine($"return new {this.Name}(value);");
+        }
+    }
 }

src/Tests/FlatSharpEndToEndTests/UnionImplicitOperator/UnionImplicitOperator.cs

@@ -0,0 +1,50 @@
+namespace FlatSharpEndToEndTests.UnionImplicitOperator;
+
+[TestClass]
+public class UnionImplicitOperatorTests
+{
+    [TestMethod]
+    public void ImplicitOperator_Struct()
+    {
+        A a = new A { V = 1 };
+
+        StructUnion u = a;
+
+        Assert.AreEqual(u.GetType(), typeof(StructUnion));
+        Assert.AreEqual(a.V, u.A.V);
+    }
+
+    [TestMethod]
+    public void ImplicitOperator_ValueStruct()
+    {
+        Vec3 v = new Vec3 { X = 1, Y = 2, Z = 3 };
+
+        StructUnion u = v;
+
+        Assert.AreEqual(u.GetType(), typeof(StructUnion));
+        Assert.AreEqual(v.X, u.Vec3.X);
+    }
+
+    [TestMethod]
+    public void ImplicitOperator_Table()
+    {
+        MyTable t = new MyTable { Field = "hello", Value = 42 };
+
+        TableUnion u = t;
+
+        Assert.AreEqual(u.GetType(), typeof(TableUnion));
+        Assert.AreEqual(t.Field, u.MyTable.Field);
+        Assert.AreEqual(t.Value, u.MyTable.Value);
+    }
+
+    [TestMethod]
+    public void ImplicitOperator_Vector()
+    {
+        VectorTable vt = new VectorTable { InnerVector = new[] { "A", "B", "C" } };
+
+        TableUnion u = vt;
+
+        Assert.AreEqual(u.GetType(), typeof(TableUnion));
+        Assert.AreEqual(vt.InnerVector, u.VectorTable.InnerVector);
+    }
+}

src/Tests/FlatSharpEndToEndTests/UnionImplicitOperator/UnionImplicitOperator.fbs

@@ -0,0 +1,25 @@
+attribute "fs_serializer";
+attribute "fs_valueStruct";
+
+namespace FlatSharpEndToEndTests.UnionImplicitOperator;
+
+struct A { V : uint; }
+
+table MyTable {
+	Field : string;
+	Value : uint;
+}
+
+struct Vec3 (fs_valueStruct) {
+	X : uint;
+	Y : uint;
+	Z : uint;
+}
+
+table VectorTable {
+	InnerVector : [ string ];
+}
+
+union StructUnion { A, Vec3 }
+
+union TableUnion { MyTable, VectorTable }