how to return and jum in other class for popup box in c#

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C#

Hi Good morning,
i am trying to stop execution after i get popup.

i have on class1.cs

in that i have
public void checkcondition()
{
if(checkcondition)
{
return true;
}
if(condition)
{
return false;
}

}

and i have window1.cs.[design]

there i wrote messagebox.show("Error");

after checking first condition i want to jump for pop up in this class. and want to stop further execution.

can somebody help me.