Add C# development environment

csharp/.envrc

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+use flake .

csharp/flake.lock

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+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1678901627,
+        "narHash": "sha256-U02riOqrKKzwjsxc/400XnElV+UtPUQWpANPlyazjH0=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "93a2b84fc4b70d9e089d029deacc3583435c2ed6",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1679163677,
+        "narHash": "sha256-VC0tc3EjJZFPXgucFQAYMIHce5nJWYR0kVCk4TVg6gg=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c3912035d00ef755ab19394488b41feab95d2e40",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "release-22.11",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

csharp/flake.nix

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+{
+  description = "A Nix-flake-based C# development environment";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/release-22.11";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs =
+    { self
+    , nixpkgs
+    , flake-utils
+    }:
+
+    flake-utils.lib.eachDefaultSystem (system:
+    let
+      pkgs = import nixpkgs { inherit system; };
+    in
+    {
+      devShells.default = pkgs.mkShell {
+        packages = with pkgs; [
+          #dotnet-sdk_6
+          dotnet-sdk_7
+          #dotnet-sdk_8
+          omnisharp-roslyn
+          mono
+          msbuild
+        ];
+      };
+    });
+}