How to validate field on form

Hi,

I want to check a value of a field in a form if it is available or not. 
I want to make a comparison of this value with other existing values,
if this value is not equal to any value the system displays the nearest.
I use the validateMethod of field in datasource like this :

public boolean validate()
{
int _lenght,counter;
;
ret = super();

_lenght= conLen(longueurs);
for (counter =1; counter <=_lenght; counter++)
{
if( _salesline.ML==conPeek(longueurs,counter))
ret=true;
info(conPeek(longueurs,counter));
}
if(ret==false)
{
Box::warning("depth is not available");
}
}

return ret;
}

I want to verifiy "_salesline.ML"

But i don't get any thing.