BackgroundWorker

static BackgroundWorker bw = new BackgroundWorker();
static BackgroundWorker bw1 = new BackgroundWorker();
static BackgroundWorker bw2 = new BackgroundWorker();
static BackgroundWorker bw3 = new BackgroundWorker();
static BackgroundWorker bw4 = new BackgroundWorker();
static BackgroundWorker bw5 = new BackgroundWorker();
 
bw = new BackgroundWorker();
bw.WorkerReportsProgress = true;
bw.WorkerSupportsCancellation = true; 
bw.DoWork += ViewMyTextBoxContents;
bw.ProgressChanged += bw_ProgressChanged;
bw.RunWorkerCompleted += bw_RunWorkerCompleted;         
bw.RunWorkerAsync(null);
 
// и так для каждого...

BackgroundWorker[] bw = new BackgroundWorker[5];
 
for(int i=0; i<bw.Lenght; i++)
{
bw[i] = new BackgroundWorker();
bw[i].WorkerReportsProgress = true;
bw[i].WorkerSupportsCancellation = true; 
bw[i].DoWork += ViewMyTextBoxContents;
bw[i].ProgressChanged += bw_ProgressChanged; // если для всех работников должен быть один обработчик этого события
bw[i].RunWorkerCompleted += bw_RunWorkerCompleted; // то же самое
bw[i].RunWorkerAsync(null);
 
}