I tried exactly your code:
TextBox txtcoeffciantA = new TextBox();
txtcoeffciantA.Text = "0.00051368";
TextBox txtcoeffciantB = new TextBox();
txtcoeffciantB.Text = "-15";
TextBox txtcoeffciantC = new TextBox();
txtcoeffciantC.Text = "1.89574224";
double a = double.Parse(txtcoeffciantA.Text);
double b = double.Parse(txtcoeffciantB.Text);
double c = double.Parse(txtcoeffciantC.Text);
Console.WriteLine("{0}:{1}:{2}", a, b, c);
And it worked exactly as I would expect:
0.00051368:-15:1.89574224
So put a breakpoint on the first Parse line, and examine your inputs. One of them is not what you think it is...