Make a C# REPL for your Business in 2 minutes

Does your Business have a .NET codebase? Did you know that You can spin up an Administrator CLI that lets you write C# expressions using the service classes and methods you know so well.

What is Mykeels.CSharpRepl?

Mykeels.CSharpRepl is a powerful, embeddable C# REPL (Read-Eval-Print Loop) for .NET applications. It lets you spin up an interactive C# environment—right inside your business app—so you (or your team) can run C# expressions, scripts, and even invoke your own service classes and methods on the fly.

This is perfect for admin CLIs, debugging, or building custom automation tools for your business. It supports syntax highlighting, code completion, custom output formatting, and more.

Getting Started

Add the NuGet package to your project:

dotnet add package Mykeels.CSharpRepl

Quick Start

To launch a REPL in your app, just add:

using Mykeels.CSharpRepl;

await Repl.Run();

This starts an interactive C# REPL with sensible defaults.

Every C# expression entered is evaluated and the result is displayed.

You can chain C# commands.

Async C# expressions work as well.

Customizing the REPL

You can customize references, namespaces, output formatting, and more by passing a Configuration object:

using CSharpRepl.Services;
using Mykeels.CSharpRepl;
using Spectre.Console;

await Repl.Run(
    new Configuration(
        // Add references to assemblies
        references: AppDomain.CurrentDomain.GetAssemblies().Select(a => $"{a.GetName().Name}.dll").ToArray(),
        // Add default namespaces
        usings: [
            "System",
            "System.Collections.Generic",
            "System.IO",
            "System.Linq",
            "System.Net.Http",
            "System.Threading",
            "System.Threading.Tasks"
        ],
        // Set application name
        applicationName: "MyApp.CSharpRepl",
        // Customize success output
        logSuccess: (message, result) => {
            Console.WriteLine($"<< {message}");
            string output = Newtonsoft.Json.JsonConvert.SerializeObject(result);
            AnsiConsole.MarkupLine($"[green]>> {output}[/]");
        },
        // Customize error output
        logError: (message, exception, _) => {
            Console.WriteLine($"<< {message}");
            string output = Newtonsoft.Json.JsonConvert.SerializeObject(new { Error = exception });
            Console.WriteLine($">> {output}");
            AnsiConsole.MarkupLine($"[red]>> {output.EscapeMarkup()}[/]");
        }
    )
);

Pre-execution Commands

You can run commands before the REPL starts—great for importing commonly used types or setting up your environment:

await Repl.Run(
    commands: [
        // Import ScriptGlobals to make its methods available directly
        "using static Mykeels.CSharpRepl.Sample.ScriptGlobals;"
    ]
);

ScriptGlobals

You can expose your own static methods and properties to the REPL by adding a static class (e.g., ScriptGlobals). Then, import it with a pre-execution command:

"using static Mykeels.CSharpRepl.Sample.ScriptGlobals;"

This makes your business logic instantly available in the REPL.

Running as an MCP Server

You can also launch an MCP server, which lets you list members of your ScriptGlobals class and invoke C# code remotely (e.g., from tools like Cursor):

await McpServer.Run(typeof(ScriptGlobals));

Best Practices

• Assembly References: Include all necessary assemblies in the references array
• Namespaces: Add commonly used namespaces to the usings array
• Error Handling: Implement custom error handling in logError for better debugging
• Output Formatting: Use AnsiConsole for colored output and better readability
• Pre-execution Commands: Use commands to set up your environment and import commonly used types

Conclusion

With Mykeels.CSharpRepl, you can empower your business with a flexible, interactive C# CLI in minutes. Whether for admin tasks, debugging, or automation, it's a game-changer for .NET teams.

Check out the GitHub repo for more details, and start building your own REPL today!

What will you build with this?