Unable to assign value to a textbox property in c#

Hi,

string stEx = cr.Text;
txtbox.Paragraphs[0].TextRuns[0].Value.Expression = stEx;

I have a value in stEx and I need to assign it to `

txtbox.Paragraphs[0].TextRuns[0].Value.Expression

`Now I am getting the below error.

Cannot modify the return value of TextRun.Value because it is not a variable

Really appreciate any suggestions on this.