Why not dynamic char array sizing

[thomas49th]

Just wondering about creating a dynamic string array

int i = this->txtMain->Text->Length - 1;
char x[i];

This gives me 3 errors

Error 1 error C2057: expected constant expression
Error 2 error C2466: cannot allocate an array of constant size 0
Error 3 error C2133: 'x' : unknown size

Can someone explain to me why. Im a n00b

Thanks