c# - How to add additional validation to Model field? -

i have second thing:

<td>     @html.labelfor(m => m.number, titlehtmlattrs) </td> <td>     <span class="element-value2">         @html.editorfor(m => m.number)         @html.validationtooltipfor(m => m.number)     </span> </td> 

and how field looks in model:

[display(name = "special number")] [stringlength(20)] public string number { get; set; } 

which means if wanted change field, can have value empty 20. it's ok, need additional validation. in model have fields:

public datetime? timeof { get; set; } public bool hastype { get; set; } 

new validation should work only if timeof not null , hastype true. new validation should prevent empty values in number. basically, change (from empty 20) (from 1 20).

how correctly accomplish this?

p.s sorry bad english.

for complex validation logic, @ implementing ivalidatableobject in viewmodel , can place conditional validation logic inside validate method. (caveat, server side)

public ienumerable<validationresult> validate(validationcontext validationcontext) {     if (this.hastype)     {        // other conditional validation        if (validationfails)        {            yield return new validationresult("descriptive error goes here");        }     }      // other validation here.