how to use datatable.compute in runtime?

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C#

Hi,
I have 2 type's of codes that use datatable in order to evaluate
a string like "2+3/4" and return the result.
I'm looking for a way to modify them so i can evaluate a string
with variable's so i can read new data from the user in runtime, like "2+X/4"
when X is assigned with some number before.

first code type uses datatable.compute:

using (DataTable eval = new DataTable())
{
    //Evaluate the Arithmetic expression
    var result = eval.Compute("1+2*3/4", null);

    //Displaying the result
    Console.WriteLine("Result: {0} of Type: {1}", result, result.GetType().Name);
}

second code:

static double Evaluate(string expression) {
  var loDataTable = new DataTable();
  var loDataColumn = new DataColumn("Eval", typeof (double), expression);
  loDataTable.Columns.Add(loDataColumn);
  loDataTable.Rows.Add(0);
  return (double) (loDataTable.Rows[0]["Eval"]);
}

doesnt matter to me which code to use.
I cant use third party projects like Ncalc,IT department doesnt allow it.