Xamarin android save text file

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C#

Android

Xamarin

i am using xamarin and i want to save txt file there so i have some script that is saving

private void SavetoSd()
       {
           var sdCardPath = Android.OS.Environment.ExternalStorageDirectory.Path;
           var filePath = System.IO.Path.Combine(sdCardPath, "iootext.txt");
           if (!System.IO.File.Exists(filePath))
           {
               using(System.IO.StreamWriter write = new System.IO.StreamWriter(filePath,true))
               {
                   write.Write(etSipServer.ToString());
               }
           }

       }

but the error is that

System.UnauthorizedAccessException: Access to the path "/mnt/sdcard/iootext.txt" is denied.

any help??

What I have tried:

in manifest i have added this

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
	<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />