How to resolve the following error in a C# console application using VS2010: "The process cannot access the file 'error.txt' because it is being used by another process."

I am developing a C# console application using VS2010 and in the following snippet of code, when the following statement is executed:

 File.Delete(errorFilePath)

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I get the error:
"The process cannot access the file 'L:\\F\FS\\C\\TEST\\error.txt' because it is being used by another process."

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static void Main()
        {            
            string errorFilePath = System.IO.Path.Combine(MyGlobals.BASE_DIR, "error.txt");
           
            //delete the error file if it presently exists so that a new error file can be created in the event any errors are discovered
            if (File.Exists(errorFilePath))
            {
                File.Delete(errorFilePath);
            }            

            DirectoryInfo parentDirectory = new DirectoryInfo(MyGlobals.BASE_DIR);

            foreach (FileInfo file in parentDirectory.GetFiles())
            {
                if (!string.Equals(file.Name, MyGlobals.BASE_FILE_Name + ".ard", StringComparison.InvariantCultureIgnoreCase) &&
                    !string.Equals(GetFileTitleOnly(file.Name), GetFileTitleOnly(currentRunningAssembly.ManifestModule.Name), StringComparison.InvariantCultureIgnoreCase))
                {
                    StreamWriter sw1 = new StreamWriter(errorFilePath);
                    ProcessFile(file.FullName, sw1);               
                }
            }

            //if an Error file was created but no information was written to it, then the error file will be deleted 
            //before the application is finished and closes down
            if (File.Exists(errorFilePath))
            {
                if (new FileInfo(errorFilePath).Length == 0)
                {
                    if (File.Exists(errorFilePath))
                    {
                        File.Delete(errorFilePath);       <---- Error occurs here
                    }
                }
            }
           
        }

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