Roslyn Scripting Syntax Transformation

As you know that syntax trees are immutable, so instead of modifying existing syntax trees, we need to create new trees using transformations when need to modify them.

The SyntaxFactory class provides factory methods to create SyntaxNodes for each kind of node, token, or trivia which we can use to create an instance of that type.

The NameSyntax is the base class for the following four types.

Let's consider the following simple example code, which contains various using directives at the top of the source code.

using System;
using System.Collections;
using System.Linq;
using System.Text;
using Microsoft.CodeAnalysis;
 
namespace HelloWorld
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(""Hello, World!"");
        }
    }
}

As you can see, the last using directive is Microsoft.CodeAnalysis;, now we will replace this using directive with Microsoft.CodeAnalysis.CSharp.Scripting; using the transformation.

The following code first creates a NameSyntax node using the IdentifierName and QualifiedName methods for the using directive which we will replace.

static void SyntaxTransformationExample()
{
    NameSyntax name = SyntaxFactory.IdentifierName("Microsoft");
    name = SyntaxFactory.QualifiedName(name, SyntaxFactory.IdentifierName("CodeAnalysis"));
    name = SyntaxFactory.QualifiedName(name, SyntaxFactory.IdentifierName("CSharp"));
    name = SyntaxFactory.QualifiedName(name, SyntaxFactory.IdentifierName("Scripting"));

    Console.WriteLine(name);

    SyntaxTree tree = CSharpSyntaxTree.ParseText(
@"using System;
using System.Collections;
using System.Linq;
using System.Text;
using Microsoft.CodeAnalysis;
 
namespace HelloWorld
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(""Hello, World!"");
        }
    }
}");

    var root = (CompilationUnitSyntax)tree.GetRoot();

    var oldUsing = root.Usings[4];
    var newUsing = oldUsing.WithName(name);

    root = root.ReplaceNode(oldUsing, newUsing);

    Console.WriteLine(root);
}

Once the source code is parsed, then we replaced the last using directive with the new one. Let's execute the above code and you will see the following output.

using System;
using System.Collections;
using System.Linq;
using System.Text;
using Microsoft.CodeAnalysis.CSharp.Scripting;

namespace HelloWorld
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello, World!");
        }
    }
}

As you can see the last using directive is updated to Microsoft.CodeAnalysis.CSharp.Scripting;.