Data Annotations for validation, at least one required field?

I have extended Zhaph answer to support grouping of properties.

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]public class AtLeastOnePropertyAttribute : ValidationAttribute{    private string[] PropertyList { get; set; }    public AtLeastOnePropertyAttribute(params string[] propertyList)    {        this.PropertyList = propertyList;    }    //See http://stackoverflow.com/a/1365669    public override object TypeId    {        get        {            return this;        }    }    public override bool IsValid(object value)    {        PropertyInfo propertyInfo;        foreach (string propertyName in PropertyList)        {            propertyInfo = value.GetType().GetProperty(propertyName);            if (propertyInfo != null && propertyInfo.GetValue(value, null) != null)            {                return true;            }        }        return false;    }}

Usage:

[AtLeastOneProperty("StringProp", "Id", "BoolProp", ErrorMessage="You must supply at least one value")]public class SimpleTest{    public string StringProp { get; set; }    public int? Id { get; set; }    public bool? BoolProp { get; set; }}

And if you want to have 2 groups (or more):

[AtLeastOneProperty("StringProp", "Id", ErrorMessage="You must supply at least one value")][AtLeastOneProperty("BoolProp", "BoolPropNew", ErrorMessage="You must supply at least one value")]public class SimpleTest{    public string StringProp { get; set; }    public int? Id { get; set; }    public bool? BoolProp { get; set; }    public bool? BoolPropNew { get; set; }}

I'd create a custom validator for this - it won't give you client side validation, just server side.

Note that for this to work, you'll need to be using nullable types, as value types will default to 0 or false:

First create a new validator:

using System.ComponentModel.DataAnnotations;using System.Reflection;// This is a class-level attribute, doesn't make sense at the property level[AttributeUsage(AttributeTargets.Class)]public class AtLeastOnePropertyAttribute : ValidationAttribute{  // Have to override IsValid  public override bool IsValid(object value)  {    //  Need to use reflection to get properties of "value"...    var typeInfo = value.GetType();    var propertyInfo = typeInfo.GetProperties();    foreach (var property in propertyInfo)    {      if (null != property.GetValue(value, null))      {        // We've found a property with a value        return true;      }    }    // All properties were null.    return false;  }}

You can then decorate your models with this:

[AtLeastOneProperty(ErrorMessage="You must supply at least one value")]public class SimpleTest{    public string StringProp { get; set; }    public int? Id { get; set; }    public bool? BoolProp { get; set; }}

Then when you call ModelState.IsValid your validator will be called, and your message will be added to the ValidationSummary on your view.

Note that you could extend this to check for the type of property coming back, or look for attributes on them to include/exclude from validation if you want to - this is assuming a generic validator that doesn't know anything about the type it's validating.

This question is pretty old, but as of .NET 3.5 (I believe), IValidatableObject can help with tricky validation situations. You can implement it to validate arbitrary business rules. In this case, something like:

public IEnumerable<ValidationResult> Validate(ValidationContext validationContext){    if (string.IsNullOrWhiteSpace(FieldOne) && string.IsNullOrWhiteSpace(FieldTwo))        yield return new ValidationResult("Must provide value for either FieldOne or FieldTwo.", new string[] { "FieldOne", "FieldTwo" });}