asp.net (C #) limits the user's code to enter specified characters and numbers

Here's the code:
Allow only user name input: User name must begin with 0~9, a~z or A~z!
Copy CodeThe code is as follows:
protected void Button3_Click (object sender, EventArgs e)
{
int error_count = 0; Legality used to identify the user name
String str = TextBox1.Text.Trim ();
if (str = string. Empty)
{
Response.Write ("User name cannot be empty!") ");
Return
}
str = str. Substring (0, 1); User name 1th character
String Strchar = "0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,a,b,c,d,e,f,g,h,i,j,k,l , m,n,o,p,q,r,s,t,u,v,w,x,y,z ";
string[] Vcarray = Strchar. Split (', ');
for (int i = 0; i < vcarray.length; i++)
{
if (str!= vcarray[i])
{
error_count++;
}
Else
{
Error_count = 0; If the user name is valid, initialize the variable error_count to 0,
Break
}
}
if (Error_count > 0)//If the variable error_count is greater than 0, the user name is illegal.
{
Response.Write (' <script>alert (' Beginning of user name, must be 0~9, a~z, or a~z! ') </script> ");
}
Else
{
Response.Write ("<script>alert" (' Legal user name, can be used!) ') </script> ");
}
}