Assuming your class currently looks something like this:

    [ValidateThisForm]
    public class MyViewModel
    {
        public bool TestOne { get; set; }
        public bool TestTwo { get; set; }
    }

If you have complex model-level validation you want to perform, where you can't simply annotate a single property, you can hook into the validation pipeline with [`IValidatableObject`][1] like this:

    public class MyViewModel : IValidatableObject
    {
        public bool TestOne { get; set; }
        public bool TestTwo { get; set; }
        
        public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
        { 
          if (TestOne == false && TestTwo == false) 
          {
              yield return new ValidationResult("Form Incomplete: Please correct")
          }
        }
    }

You can continue to `yield return` as many validation messages as you'd like. And you can display them all on the client with [`@Html.ValidationSummary`][3]

If you'd like the message to appear alongside a particular control, the [`ValidationResult`][2] constructor takes an overload with the `memberNames` of the affected properties so you could change the `Validate` method like this:

    if (TestOne == false && TestTwo == false) 
    {
        yield return new ValidationResult("Form Incomplete Message", new[] {"TestOne"})
    }

Then you can provide the validation message for that particular property with the [`ValidationMessageFor`][4] HTML helper like this:

    @Html.ValidationMessageFor(Function(model) model.TestOne )

For further reading, Scott Guthrie wrote about using `IValidatableObject` as part of his [blog post introducing new features in MVC3][5] and had a follow up post with more detail on [class level validation][6] shortly thereafter.

 
[1]: https://msdn.microsoft.com/en-us/library/system.componentmodel.dataannotations.ivalidatableobject%28v=vs.110%29.aspx
[2]: https://msdn.microsoft.com/en-us/library/dd411880(v=vs.110).aspx
[3]: https://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.validationsummary%28v=vs.110%29.aspx
[4]: https://msdn.microsoft.com/en-us/library/system.web.mvc.html.validationextensions.validationmessagefor%28v=vs.118%29.aspx
[5]: http://weblogs.asp.net/scottgu/introducing-asp-net-mvc-3-preview-1
[6]: http://weblogs.asp.net/scottgu/class-level-model-validation-with-ef-code-first-and-asp-net-mvc-3

I have a viewmodel such that, on validation, I want to compare multiple fields. 
I have a custom attribute that takes the viewmodel and performs the required validation.
I am decorating the viewmodel class with the custom attribute [ValidateThisForm]

    [AttributeUsage(AttributeTargets.Class)]
    public class ValidateThisForm : ValidationAttribute
    {
        public override bool IsValid(object value)
        {
            ViewModel _viewModel = value as ViewModel;

            if (_viewModel.TestOne == false && _viewModel.TestTwo == false)
            {
                ErrorMessage = "Form Incomplete: Please correct";
                return false;
            }
            else
            {
                return true;
            }
        }
    }


The problem is, I want to perform several "class level" validations. As a result, in my view, I can only display these errors in one place on the form:

    <td class = "field-validation-error">@Html.ValidationMessageFor(viewmodel => viewmodel)</td>

Is there a way that I can have multiple "class level" errors being displayed in different locations on the form? 


MVC - Displaying multiple class level validation error messages

That window is a pita but there isn't anything you can do to stop it from showing up.  

Note how the original TabPage control doesn't show up in the toolbox, it is only ever added by the custom designer for TabControl.  This is done by giving it the [ToolboxItem(false)] attribute.  And your nemesis, [Designer] attributes that select custom designers for both the TabControl and the TabPage classes.  You'd have to create your own ExtendedTabControl and make a designer for it.  This is not fun, the TabControlDesigner class is a rather elaborate designer.  It is also *internal* so you cannot derive from it.  Using a decompiler like Reflector or ILSpy is a way to peek at the code in those designers.  I cannot recommend this unless you have a large and demanding user base of picky paying programmers.

Writing code instead of trying to make this work at design time is feasible.  But do note that you have to TryCast() at runtime to convert the TabPage reference you get from the TabPages property to your derived class.

I'm trying to use the [`System.ComponentModel.DesignerCategory`](http://msdn.microsoft.com/en-us/library/system.componentmodel.designercategoryattribute.aspx) Attribtue to prevent an extended Tab Page componenet from automatically opening up in the designer.   The suggestions on other StackOverflow questions [1][Q1],[2][Q2],[3][Q3], don't seem to work.

**Here's the code on the component**




    <System.ComponentModel.DesignerCategory("Code")>
    Public Class ExtendedTabPage : Inherits Windows.Forms.TabPage

**Here's the code in my .vbproj file**





    <Compile Include="ExtendedTabPage.vb">
      <SubType>Component</SubType>
    </Compile>

I've tried initializaing the DesignerCategory Attribute with a `""` and also `"Code"`, doing rebuilds and reopening the solution, but I still get the screen below from  a double click on the component.  Do I have to remove the project SubType?  Will that affect the way the code compiles?


![Component Designer][1]


  [1]: http://i.stack.imgur.com/BjBil.png
  [Q1]: https://stackoverflow.com/questions/2138684/is-it-possible-to-tell-visual-studio-not-to-treat-a-source-file-as-a-component
  [Q2]: https://stackoverflow.com/questions/230146/is-there-an-attribute-i-can-add-to-a-class-so-it-will-be-edited-as-code-not-in
  [Q3]: https://stackoverflow.com/questions/1789603/visual-studio-make-view-code-default-using-attribute