nullabledatetimepicker

我的MainForm.Designer.cs中代码是这样的
this.creationEndDate = new iFramework.Forms.NullableDateTimePicker();
this.dateSearchGroupBox.Controls.Add(this.creationEndDate);this.creationEndDate.Format = System.Windows.Forms.DateTimePickerFormat.Custom;
this.creationEndDate.Location = new System.Drawing.Point(164,28);
this.creationEndDate.Name = "creationEndDate";
this.creationEndDate.Size = new System.Drawing.Size(84,20);
this.creationEndDate.Value = new System.DateTime(2006,10,20,15,32,30,328);
this.creationEndDate.KeyDown += new System.Windows.Forms.KeyEventHandler(this.creationEndDate_KeyDown);
Private iFramework.Forms.NullableDateTimePicker creationEndDate;但是我的MainFrom.cs[Design]打开时出现这样的错误的："Could not find type 'iFramework.Forms.NullableDateTimePicker'.Please make sure that the assembly that contains this type is referenced. If this type is a part of your development project,make sure that the project hat been successfully built.""The variable 'creationEndDate' is either underclared or was never assigned"请高手执教，谢谢。