my c# calculator only accepts one digit after selecting an o

Question

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using System.Windows.Forms;

namespace WindowsFormsApplication1

{

public partial class CALCULATOR : Form

{

Double ResultsCalur = 0;

string operationPerformed = "";

bool isOperationPerformed = false;

public CALCULATOR()

{

InitializeComponent();

}

private void button14_Click(object sender, EventArgs e)

{

}

private void CALCULATOR_Load(object sender, EventArgs e)

{

}

private void txtdivide_Click(object sender, EventArgs e)

{

}

private void btnce_Click(object sender, EventArgs e)

{

textBox1.Text = "0";

}

private void btnc_Click(object sender, EventArgs e)

{

textBox1.Text = "0";

ResultsCalur = 0;

}

private void txtequals_Click(object sender, EventArgs e)

{

switch (operationPerformed)

{

case "+":

textBox1.Text = (ResultsCalur + Double.Parse(textBox1.Text)).ToString();

break;

case "-":

textBox1.Text = (ResultsCalur - Double.Parse(textBox1.Text)).ToString();

break;

case "/":

textBox1.Text = (ResultsCalur / Double.Parse(textBox1.Text)).ToString();

break;

case "*":

textBox1.Text = (ResultsCalur * Double.Parse(textBox1.Text)).ToString();

break;

}

ResultsCalur = Double.Parse(textBox1.Text);

lblcurrentopp.Text = " ";

}

private void btn0_Click(object sender, EventArgs e)

{

if ((textBox1.Text == "0") || (isOperationPerformed))

textBox1.Clear();

Button button = (Button)sender;

if(button.Text ==".")

{

if (!textBox1.Text.Contains("."))

textBox1.Text = textBox1.Text + button.Text;

}else

textBox1.Text = textBox1.Text + button.Text;

}

private void txtplus_Click_1(object sender, EventArgs e)

{

Button button = (Button)sender;

if(ResultsCalur != 0){

txtequals.PerformClick();

operationPerformed = button.Text;

lblcurrentopp.Text = ResultsCalur + " " + operationPerformed;

isOperationPerformed = true;

}

else {

operationPerformed = button.Text;

ResultsCalur = Double.Parse(textBox1.Text);

lblcurrentopp.Text = ResultsCalur + " " + operationPerformed;

isOperationPerformed = true;

}

}

}

}