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.
You could probably create your own control which inherits from TextBox and then override Text property to automaticaly add °C though other color inside the same TextBox could be problem. 

Why you want to have °C in TextBox ? 
Can't it just be label right after TextBox ? 
You can set static text and color to what you want.