How can I replace a string in a file?

I have a batch file as below:
call "C:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\Common7\Tools\VsDevCmd.bat"
cd C:\AutomationTestSamples\AutomationTestSamples\bin\Debug
start cmd /k vstest.console.exe AutomationTestSamples.dll /Logger:trx;LogFileName=%1/UnitTest1.trx /testcasefilter:FullyQualifiedName~AutomationTestSamples.UnitTest1.trx

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I want to read a batch file line by line and if the line contains "UnitTest1.trx",
I want to replace "/testcasefilter:FullyQualifiedName~AutomationTestSamples.UnitTest1" with "/test".

I tried the following code but when I opened the file nothing was replaced.  Any idea please?
string[] str = File.ReadAllLines(@"C:\Test\test.bat");
            for (int i = 0; i < str.Length; i++)
                if (str[i].Contains(trxName))
                    str[i].Replace(str[i].Substring(str[i].IndexOf("/testcasefilter")), "/test");                    

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Chinmay PatelChief Technology NinjaCommented:
Hi Isabell,

In the code sample above you are just reading the file contents in an string array. You need to write back those lines to the file as well to get the desired results.

Also string.Replace returns a string value which you have to use. It does not actually replace/change the content of the string.

Chinmay PatelChief Technology NinjaCommented:
str[i].Replace(str[i].Substring(str[i].IndexOf("/testcasefilter")), "/test");                    

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should be
str[i] = str[i].Replace(str[i].Substring(str[i].IndexOf("/testcasefilter")), "/test");                    

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And after your For loop has ended, use File.WriteAllLines method

For example,
File.WriteAllLines(@"C:\Test\test.bat", str);

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IsabellAuthor Commented:
Thanks Chinmay,
That was a perfect solution!
Chinmay PatelChief Technology NinjaCommented:
Glad I could help.
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