Question

textBlock.Text = "Text";

This is my code, and it shows no errors.
but when I run it, I get a NullReferenceException

Object reference not set to an instance of an object.

This statement is inside a ValueChanged event of a Slider, should it matter.

Answer 1

I assume this code is in your constructor. Make sure InitializeComponents is called before you execute this line:

public YourWindow()
{
    TextBlock.Text = "Text"; // <- bad
    InitializeComponents();
    TextBlock.Text = "Text"; // <- good
}