var obj = new Foo("bar", "baz", thirdParam: 5);

var arg1 = new CodePrimitiveExpression("bar");
var arg2 = new CodePrimitiveExpression("baz");
var arg3 = new CodePrimitiveExpression(5);

var objectCreate = new CodeObjectCreateExpression(typeof(Foo), arg1, arg2, arg3);

var decl = new CodeVariableDeclarationStatement("var", "obj", objectCreate);

var obj = new Foo("bar", "baz", 5);