C# need help with stringreader readtoend method

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C#

I am trying to read all content as a string using StringReader.ReadToEnd method
To me my solution seems correct but Unit test does not pass and I could not figure out why. Any help would be appreciated.

When I debugged it I got this error:

Expected: "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed ..."
  But was:  "System.IO.StringReader"

What I have tried:

public static string ReadFullContent(StringReader stringReader)
        {
           
            StringReader reader = new StringReader(stringReader.ToString());

            return reader.ReadToEnd();
        }