.NET API Simulator

Endpoint

using System;

public class Program {
    public static void Main(string[] args) {
        string body = Console.In.ReadToEnd();
        string name = "guest";

        if (!string.IsNullOrEmpty(body)) {
            string marker = "\"name\"";
            int idx = body.IndexOf(marker, StringComparison.OrdinalIgnoreCase);
            if (idx >= 0) {
                int colon = body.IndexOf(':', idx + marker.Length);
                if (colon >= 0) {
                    int firstQuote = body.IndexOf('"', colon + 1);
                    if (firstQuote >= 0) {
                        int secondQuote = body.IndexOf('"', firstQuote + 1);
                        if (secondQuote > firstQuote) {
                            name = body.Substring(firstQuote + 1, secondQuote - firstQuote - 1);
                        }
                    }
                }
            }
        }

        name = name.Replace("\\", "\\\\").Replace("\"", "\\\"");

        string json = "{\n" +
            "  \"status\": \"ok\",\n" +
            "  \"message\": \"Hello " + name + "\",\n" +
            "  \"framework\": \".NET\"\n" +
            "}";

        Console.WriteLine(json);
    }
}

Request Body (JSON)

Route Args (space separated)

Response

What Is the .NET API Simulator?

This tool lets you write C# code that mimics ASP.NET-style API controller patterns and test JSON request/response handling directly in your browser. You provide a JSON request body, write a C# handler that processes it, and see the JSON response output instantly without installing ASP.NET or running a local server.

The simulator focuses on the data-processing logic inside an ASP.NET controller action. You can parse incoming JSON using C# string operations, transform the data, and print a formatted JSON response. This is the same logic you would place inside a controller action method that returns Ok() or JsonResult in a real ASP.NET application.

All code runs on a remote sandboxed server powered by the Piston API. Your browser sends the C# source code and the JSON request body to the server, which compiles and executes the program inside an isolated container and returns the printed output as the simulated API response.

How It Works

Write your C# controller logic in the editor panel. The code reads a JSON request body from standard input, processes the data, and prints a JSON response to standard output.
Enter a JSON request body in the Request Body field. This simulates the payload that would be sent to an ASP.NET API endpoint.
Add route arguments in the Route Args field if your handler expects route parameters such as a product ID or resource name.
Click the Run button to send your code and input to the execution server. The server compiles and runs the C# program with your JSON body provided through standard input.
View the response in the Response panel. The output shows whatever your handler prints, which is typically a formatted JSON response object.

Step-by-Step Example

Suppose you want to simulate an ASP.NET controller endpoint that accepts a user's name and email, then returns a registration confirmation. Here is how to do it:

First, type the following code into the editor panel:

using System;

public class Program {
    public static void Main(string[] args) {
        string body = Console.In.ReadToEnd();
        string name = "guest";
        string email = "unknown";

        if (!string.IsNullOrEmpty(body)) {
            name = ExtractValue(body, "name") ?? "guest";
            email = ExtractValue(body, "email") ?? "unknown";
        }

        Console.WriteLine("{");
        Console.WriteLine("  \"status\": \"ok\",");
        Console.WriteLine("  \"message\": \"Welcome " + name + "\",");
        Console.WriteLine("  \"email\": \"" + email + "\",");
        Console.WriteLine("  \"framework\": \".NET\"");
        Console.WriteLine("}");
    }

    static string ExtractValue(string json, string key) {
        string marker = "\"" + key + "\"";
        int idx = json.IndexOf(marker);
        if (idx < 0) return null;
        int colon = json.IndexOf(':', idx + marker.Length);
        if (colon < 0) return null;
        int q1 = json.IndexOf('"', colon + 1);
        if (q1 < 0) return null;
        int q2 = json.IndexOf('"', q1 + 1);
        if (q2 <= q1) return null;
        return json.Substring(q1 + 1, q2 - q1 - 1);
    }
}

Next, enter the following JSON in the Request Body field:

{"name": "Asha", "email": "asha@example.com"}

Now click the Run button. The server compiles and executes your C# handler with the provided JSON body. After a moment, the Response panel displays a JSON object with the registration confirmation.

You can modify the request body or the controller logic and run again to test different scenarios, just as you would iterate on a real ASP.NET controller action.

Use Cases

Learning ASP.NET controller patterns. Understand how ASP.NET controllers parse JSON request bodies and construct JSON responses without setting up a full .NET project.
Prototyping API response logic. Quickly test data transformation and validation logic in C# before implementing it in a full ASP.NET application.
Practicing JSON handling in C#. Experiment with string parsing, data extraction, and JSON response construction using C#'s standard library.
Teaching REST API concepts. Demonstrate request/response cycles, controller patterns, and JSON formatting in a classroom or tutorial setting.
Testing edge cases. Verify how your handler logic responds to missing fields, empty bodies, or unexpected data types without deploying an ASP.NET application.

Limitations and Notes

No real ASP.NET server. This is a simulation. The ASP.NET framework is not installed or running. You write handler logic in plain C#.
No Entity Framework. There is no database or Entity Framework support in the sandbox. Simulate data with C# collections and objects instead.
No dependency injection. The ASP.NET DI container, constructor injection, and service registration are not available.
No NuGet packages. NuGet packages and external libraries are not available. Only the .NET standard library can be used.
No middleware. ASP.NET middleware pipelines, authentication handlers, and authorization policies are not available.
Single file execution. Your entire handler must be in one C# file. Multi-file projects and namespace imports beyond the standard library are not supported.
Execution timeout applies. Programs that run too long or enter infinite loops are terminated automatically.

Frequently Asked Questions

Does this run a real ASP.NET server?

No. This is a simulation that runs C# code to mimic ASP.NET-style controller patterns. No actual ASP.NET server or Kestrel host is started.

Does Entity Framework work here?

No. Entity Framework and database connections are not available. You can simulate data using C# collections but cannot connect to any database.

Does dependency injection work?

No. The ASP.NET dependency injection container is not available. You write plain C# code without constructor injection or service registration.

Do routing attributes work?

Routing attributes like [Route] and [HttpGet] are shown as a simulated pattern for learning purposes. The tool does not process actual ASP.NET attribute routing.

Can I install NuGet packages?

No. NuGet packages and external libraries are not available. Only the .NET standard library classes can be used in your code.

Does middleware work?

No. ASP.NET middleware pipelines are not available in the sandbox. You write plain C# handler logic without middleware processing.

What is the main use case for this tool?

It is designed for learning ASP.NET-style API controller patterns, practicing JSON request and response handling in C#, and understanding how controllers process data.

Can I return JSON responses?

Yes. You can construct JSON strings in C# and print them to standard output, simulating the JSON response an ASP.NET controller would return with Ok() or JsonResult.