data conversion problem in C#

I'm having trouble following your business logic. You start out by declaring a variable a as a double. Then you test the txtItemQty value to see if it is blank, and if it is, you immediately recast the type to float and assign it the value of 0.00. Later you clearly indicate that you mean the variable m to hold the value of txtItemQty as a double; why not be consistent?

Let's do this another way:

if (txtItemQty.Text != "")
   {
      double m = double.Parse(txtItemQty.Text);
      double n = double.Parse(txtPrice.Text);
      double k = m*n;
      MessageBox.Show("Your cost is " & k.ToString());
   }
else
   {
      MessageBox.Show("Please enter a quantity and retry.");
   }

float.Parse() receive, string as argument pass the variable this way:

double a = 0.00;

          if (txtItemQty.Text != "")
          {
              a = float.Parse(textBox1.Text);
          }
          else
          {
              double m = double.Parse(txtItemQty.Text);
              double n = double.Parse(txtPrice.Text);
              double k = m * n;

              a = float.Parse(k.ToString()); //here conversion problem started
              //Error: float.Parse(String)has some invalid arguments

              MessageBox.Show(a.ToString());