SampleRequestValidator.cs

public class SampleRequestValidator : RequestValidator  
{
    protected override bool IsValidRequestString( HttpContext context, string value, RequestValidationSource requestValidationSource, string collectionKey, out int validationFailureIndex )
    {
        validationFailureIndex = 0;
        if ( requestValidationSource == RequestValidationSource.Form 
            && collectionKey.Equals( WSFederationConstants.Parameters.Result, StringComparison.Ordinal ) )
        {
            SignInResponseMessage message = WSFederationMessage.CreateFromFormPost( context.Request ) as SignInResponseMessage;
            if ( message != null )
            {
                return true;
            }
        }
        return base.IsValidRequestString( context, value, requestValidationSource, collectionKey, out validationFailureIndex );
    }
}