I try to convert a part of code from VBScript to C#. I have these lines :
string replacementMatch = ....; ...
replacementMatch = Eval(replacementMatch); OR
Execute(replacementMatch); I can't find the equivalent in C#.
The Eval function evaluates an expression and returns the result. You will not find an equivalent in statically typed languages such as C# and VB.NET. There are some libraries that provide such functionality such as flee.
You can add a reference to Microsoft Script Control and call its Eval function in c#. See this answer.
How can I evaluate the result of a boolean expression in string format on runtime in C#?
In the documentation VBScript you can find that "Eval" and "Execute" arguments are evaluated as "as a VBScript expression". So in short words there are no C# equivalents.
In general you should review the vb code and consider rewrite it in C# to not require evaluating script at the runtime.
If you really need to create statements at runtime and evaluate them, you could try third party tools - like "http://www.codeproject.com/KB/dotnet/evaluator.aspx" (although I didn't used this particular tool - I could see very few situations when this is really needed).