Winform Custom Control Why "Object Reference not set to an instance of an object"?

Question

I created a custom control with a max value. When adding if (DesignMode) Parent.Refresh(); it compiles but in client it crashes with the message error:

Object Reference not set to an instance of an object

Call stack:

at System.ComponentModel.ReflectPropertyDescriptor.SetValue(Object component, Object value)
at Microsoft.VisualStudio.Shell.Design.VsTargetFrameworkPropertyDescriptor.SetValue(Object component, Object value)
at System.ComponentModel.Design.Serialization.CodeDomSerializerBase.DeserializePropertyAssignStatement(IDesignerSerializationManager manager, CodeAssignStatement statement, CodePropertyReferenceExpression propertyReferenceEx, Boolean reportError)
at System.ComponentModel.Design.Serialization.CodeDomSerializerBase.DeserializeAssignStatement(IDesignerSerializationManager manager, CodeAssignStatement statement)
at System.ComponentModel.Design.Serialization.CodeDomSerializerBase.DeserializeStatement(IDesignerSerializationManager manager, CodeStatement statement) 

Source code:

[Category("Main")]
[Description("Max Value")]
[DefaultValue(100)]
public int Max
{
    get { return _max; }
    set { 
        _max = value;
        if (DesignMode)
        {
            Parent.Refresh();
        }
    }
}

Answer 1

try this:

if (DesignMode && Parent != null)
{
    Parent.Refresh();
}

Most probably the control has not yet been added to its Parent when the value is set for the first time.

If you look at your Form's *.designer.cs, you'll notice that the properties of your usercontrol get assigned before it gets added to the parent form. That's why you get the exception.