C# - Variable Returns To Its Default Value (zero) In Other Scopes?

Can I assign a value to a variable (int) and never lose this value inside any scope? the problem is I am assigning the value to the variable in some scopes but the variable returns to its default value (zero) in other scopes..

protected void Button_AddNewCourse_Click(object sender, EventArgs e)
{
ID = 10;
}

so when I am trying to use ID in other functions it falls back to zero

protected void AnotherFunction(object sender, EventArgs e)
{
// Variable ID here is zero
}