How can I execute C# code using Roslyn from a string and use global variables from a Dictionary<string, object>?

Let's break down your compilation error into two issues:

(2,26): error CS0103: The name 'x' does not exist in the current context
(2,30): error CS0103: The name 'y' does not exist in the current context
(3,16): error CS0103: The name 'message' does not exist in the current context

For these errors this is because you just defined a Dictionary named Variables in your custom Globals class. That's why when your script is being executed there are no x or y could be found as they are stored inside Variables dictionary.

To fix it you need to access them as key-value pair:

string script = @"
    var result = (int)Variables[""x""] + (int)Variables[""y""]; 
    $""{message} Result: {result}"";
";

Remember to cast it back to int otherwise it will throw as you store the values in dictionary as object:

(2,22): error CS0019: Operator '+' cannot be applied to operands of type 'object' and 'object'

Then the other issue:

(3,13): error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement

This is because of you just created a string literal without doing anything in your script:

$""{message} Result: {result}"";

To fix this, and I am assuming you are trying to print the result, change your script to:

Console.WriteLine($""{Variables[""message""]} Result: {result}"");

You will need to add the import to have Console.WriteLine() works:

var options = ScriptOptions.Default
.AddReferences(typeof(object).Assembly)
.AddImports("System"); // <- Here

Otherwise it will throw the following:

(3,9): error CS0103: The name 'Console' does not exist in the current context

The complete code will be as follow:

using Microsoft.CodeAnalysis.CSharp.Scripting;
using Microsoft.CodeAnalysis.Scripting;

var variables = new Dictionary<string, object>
    {
        { "x", 10 },
        { "y", 20 },
        { "message", "Hello, Roslyn!" }
    };

string script = @"
        var result = (int)Variables[""x""] + (int)Variables[""y""]; 
        Console.WriteLine($""{Variables[""message""]} Result: {result}"");
";

// Options de script
var options = ScriptOptions.Default
.AddReferences(typeof(object).Assembly)
.AddImports("System");

var globals = new Globals() { Variables = variables };

try
{
    var result = await CSharpScript.EvaluateAsync<string>(
        script,
        options,
        globals
    );

    Console.WriteLine(result);
    Console.ReadLine();
}
catch (CompilationErrorException ex)
{
    Console.WriteLine("Erreur de compilation :");
    Console.WriteLine(string.Join(Environment.NewLine, ex.Diagnostics));
}
Console.ReadLine();


public class Globals
{
    public Dictionary<string, object> Variables { get; set; }
}

And the output when execute:

Hello, Roslyn! Result: 30

Reference: dotnet/roslyn GitHub Repository Scripting-API-Samples

EDIT:

If the value of x and y and message is fixed, you can do the following to keep user input simple:

string preprocessing = @"
    var x = (int)Variables[""x""];
    var y = (int)Variables[""y""];
";
string script = @"
    var result = x + y; 
    Console.WriteLine($""{Variables[""message""]} Result: {result}"");
";
var builder = new StringBuilder(preprocessing);
builder.Append(script);
//...
var result = await CSharpScript.EvaluateAsync<string>(
    builder.ToString(),
    options,
    globals
);