C# REPL

C# REPL is a tool for rapid experimentation and exploration of C# expressions, statements, and NuGet packages. It provides a command line REPL for C# that supports syntax highlighting, intellisense, and type/method documentation.

This C# REPL helps you explore the C# language and libraries, including the .NET SDK and NuGet packages. It's also useful for iteratively defining and refining functions when developing software.

Installation

C# REPL is a .NET 5 global tool. It can be installed via:

dotnet tool install -g csharprepl

After installation is complete, run csharprepl to begin.

Usage:

Run csharprepl from the command line to begin an interactive session. The default colorscheme uses the color palette defined by your terminal, but these colors can be changed using a theme.json file provided as a command line argument.

Evaluating Code

Type some C# into the prompt and press Enter to run it. The result, if any, will be printed:

> Console.WriteLine("Hello World")
Hello World

> DateTime.Now.AddDays(8)
[6/7/2021 5:13:00 PM]

To evaluate multiple lines of code, use Shift+Enter to insert a newline:

> var x = 5;
  var y = 8;
  x * y
40

Additionally, if the statement is not a "complete statement" a newline will automatically be inserted when Enter is pressed. For example, in the below code, the first line is not a syntactically complete statement, so when we press enter we'll go down to a new line:

> if (x == 5)
  | // caret position, after we press Enter on Line 1

Finally, pressing Ctrl+Enter will show a "detailed view" of the result. For example, for the DateTime.Now expression below, on the first line we pressed Enter, and on the second line we pressed Ctrl+Enter to view more detailed output:

> DateTime.Now // Pressing Enter shows a reasonable representation
[5/30/2021 5:13:00 PM]

> DateTime.Now // Pressing Ctrl+Enter shows a detailed representation
[5/30/2021 5:13:00 PM] {
  Date: [5/30/2021 12:00:00 AM],
  Day: 30,
  DayOfWeek: Sunday,
  DayOfYear: 150,
  Hour: 17,
  InternalKind: 9223372036854775808,
  InternalTicks: 637579915804530992,
  Kind: Local,
  Millisecond: 453,
  Minute: 13,
  Month: 5,
  Second: 0,
  Ticks: 637579915804530992,
  TimeOfDay: [17:13:00.4530992],
  Year: 2021,
  _dateData: 9860951952659306800
}

A note on semicolons: C# expressions do not require semicolons, but statements do. If a statement is missing a required semicolon, a newline will be added instead of trying to run the syntatically incomplete statement; simply type the semicolon to complete the statement.

> var now = DateTime.Now; // assignment statement, semicolon required

> DateTime.Now.AddDays(8) // expression, we don't need a semicolon
[6/7/2021 5:03:05 PM]

Exploring Code

• Press F1 when your caret is in a type, method, or property to open its official MSDN documentation.
• Press Ctrl+F1 to view its source code on https://source.dot.net/.
• Press Ctrl+Shift+C to copy the current line of code to your clipboard, and Ctrl+v to paste.

For example, pressing F1 or Ctrl+F1 when the caret is in the AddDays function will open the MSDN documentation for AddDays and the source code for AddDays respectively.

> DateTime.Now.AddD|ays(8) // "|" denotes caret position

Adding References

Use the #r command to add assembly or nuget references.

• For assembly references, run #r "AssemblyName" or #r "path/to/assembly.dll"
• For nuget references, run #r "nuget: PackageName" to install the latest version of a package, or #r "nuget: PackageName, 13.0.5" to install a specific version (13.0.5 in this case).

To run ASP.NET applications inside the REPL, start the csharprepl application with the --framework parameter, specifying the Microsoft.AspNetCore.App shared framework. Then, use the above #r command to reference the application DLL. See the Command Line Configuration section below for more details.

csharprepl --framework  Microsoft.AspNetCore.App

Command Line Configuration

The C# REPL supports multiple configuration flags to control startup, behavior, and appearance:

Usage: csharprepl [OPTIONS] [response-file.rsp] [script-file.csx]

Supported options are:

• OPTIONS:
  • -r <dll> or --reference <dll>: Add an assembly reference. Can be specified multiple times.
  • -u <namespace> or --using <namespace>: Add a using statement. Can be specified multiple times.
  • -f <framework> or --framework <framework>: Reference a shared framework. Available shared frameworks depends on the local .NET installation, and can be useful when running a ASP.NET application from the REPL. Example frameworks are:
    • Microsoft.NETCore.App (default)
    • Microsoft.AspNetCore.All
    • Microsoft.AspNetCore.App
    • Microsoft.WindowsDesktop.App
  • -t <theme.json> or --theme <theme.json>: Read a theme file for syntax highlighting. The NO_COLOR standard is supported.
  • -v or --version: Show version number and exit.
  • -h or --help: Show this help and exit.
• response-file.rsp: A filepath of an .rsp file, containing any of the above command line options.
• script-file.csx: A filepath of a .csx file, containing lines of C# to evaluate before starting the REPL.

Integrating with other software

C# REPL is a standalone software application, but it can be useful to integrate it with other developer tools:

Windows Terminal

To add C# REPL as a menu entry in Windows Terminal, add the following profile to Windows Terminal's settings.json configuration file (under the JSON property profiles.list):

{
    "name": "C# REPL",
    "commandline": "csharprepl"
},

To get the exact colors shown in the screenshots in this README, install the Windows Terminal Dracula theme.

Visual Studio Code

To use the C# REPL with Visual Studio Code, simply run the csharprepl command in the Visual Studio Code terminal. To send commands to the REPL, use the built-in Terminal: Run Selected Text In Active Terminal command from the Command Palette (workbench.action.terminal.runSelectedText).

Comparison with other REPLs

This project is far from being the first REPL for C#. Here are some other projects; if this project doesn't suit you, another one might!

Visual Studio's C# Interactive pane is full-featured (it has syntax highlighting and intellisense) and is part of Visual Studio. This deep integration with Visual Studio is both a benefit from a workflow perspective, and a drawback as it's not cross-platform. As far as I know, the C# Interactive pane does not support NuGet packages or navigating to documentation/source code. Subjectively, it does not follow typical command line keybindings, so can feel a bit foreign.

csi.exe ships with C# and is a command line REPL. It's great because it's a cross platform REPL that comes out of the box, but it doesn't support syntax highlighting or autocompletion.

dotnet script allows you to run C# scripts from the .NET CLI. It has a REPL built-in, but the predominant focus seems to be as a script runner. It's a good alternative though, and has a strong community following.

dotnet interactive is a tool from Microsoft that creates a Jupyter notebook for C#, runnable through Visual Studio Code.