Question: I need some code to convert standard C# TextBox to temperature TextBox which means adding "°C" to end of the text in the textbox with another color than the default color.

Answer: 
To get the degree symbol you can use character code 176 e.g.

```
Char degree = (Char)176

```

You can then append this to your textbox content or I would just add a label to the right of the textbox with the degree symbol if you want to control the forecolor easily.
The other solutions proposed here are probably sufficient for your application; however, if you had the need to implement this with *re-usability* in mind, here is a custom control solution which you may extend to better suit your application:

```
public class TemperatureTextBox : ContainerControl
{
    private const int BORDER_SIZE = 1;

    // Exposes text property of text box,
    // expose other text box properties as needed:
    public override string Text
    {
        get { return textBox.Text; }
        set { textBox.Text = value; }
    }

    private TextBox textBox = new TextBox()
    {
        Text = string.Empty,
        BorderStyle = BorderStyle.None,
        Dock = DockStyle.Fill
    };

    private Label label = new Label()
    {
        Text = "°C",
        TextAlign = ContentAlignment.MiddleCenter,
        Size = new Size()
        {
            Width = 32
        },
        BackColor = SystemColors.Window,
        Dock = DockStyle.Right
    };

    public TemperatureTextBox()
    {
        this.BackColor = SystemColors.Window;
        this.Padding = new Padding(BORDER_SIZE);
        this.Controls.Add(label);
        this.Controls.Add(textBox);
        this.PerformLayout();
    }

    // Constrain control size to textbox height plus top and bottom border:
    protected override void OnResize(EventArgs e)
    {
        base.OnResize(e);
        this.Height = (textBox.Height + (BORDER_SIZE * 2));
    }

    // Render a border around the control:
    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);
        e.Graphics.DrawRectangle(
            SystemPens.ControlDarkDark,
            new Rectangle()
            {
                Width = (this.Width - BORDER_SIZE),
                Height = (this.Height - BORDER_SIZE)
            });
    }
}

```

Simply create a new class and drop this code in and rebuild you solution. It will create a new `TemperatureTextBox` control in the toolbox which can be dropped onto a new form and visually designed. 

This example exposes the `Text` property of the underlying text box by overriding the custom control's text property. You may want to expose other properties, and events depending on what your application needs to accomplish.