How can I prevent Freezing Form in Multi thread Application?

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private void BtnBFS_Click(object sender, EventArgs e)
{
Thread[] SpiderThread = new Thread[n];
object[] Param = new object[n];
ThreadCount = n;
for (int i = 0; i < n; i++)
Param[i] = TxtURL.Lines[i].ToString();
for (int Index = 0; Index < n; Index++)
{
SpiderThread[Index] = new Thread(BFS);
SpiderThread[Index].Name = Index.ToString();
}
for (int i = 0; i < n; i++)
SpiderThread[i].Start(Param[i]);
for (int i = 0; i < n n; i++)
SpiderThread[i].Join();
}
/////............................................................
public void BFS(object Param)
{
try
{
define some variable
while (UnCrawledQueue.Count >= 0 and visited <= NumC)
{
main process;
}
}
}