c# - Winform自定义控件为什么是 "Object Reference not set to an instance of an object"？

Question

我创建了一个具有最大值的自定义控件。添加时 if (DesignMode) Parent.Refresh();它可以编译，但在客户端中会因消息错误而崩溃:

Object Reference not set to an instance of an object

调用堆栈:

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) 

源代码:

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

Answer 1

试试这个:

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

当第一次设置该值时，很可能该控件尚未添加到其父级。

如果查看表单的 *.designer.cs，您会注意到用户控件的属性在被添加到父表单之前被分配。这就是您获得异常的原因。