ASP.net LINQ: return items where id equals a variable
var result = (from b in dc.Businesses
                  select b).ToList();
    return result;

var result = (from b in dc.Businesses where b.BusinessType == aVariableValue
                  select b).ToList();
    return result;

public void Button1_Click(object sender, EventArgs e)
{
    var keuze = "Nachtwinkel";
}

public IList<Business> selectAll()
{
    var result = (from b in dc.Businesses where b.BusinessType == keuze
                  select b).ToList();
    return result;
}

public void Button1_Click(object sender, EventArgs e)
{
     Session["keuze"] = "Nachtwinkel";
}

public IList<Business> selectAll()
{
    var result = (from b in dc.Businesses where b.BusinessType == Session["keuze"]
                  select b).ToList();
    return result;
}

//this line in your OnClick function
keuze = "MyBusinessTypeName";

//this line in your data handler
var result = (from b in dc.Businesses where b.BusinessType.BusinessTypeName == keuze
              select b).ToList();

public bool FilterByBusinessTypeID(dc.Business dcBusiness)
{
   return dcBusiness.ID == anIdVariable;
}
var result= BusinessType.Where(FilterByBusinessTypeID).First();

var result = (from b in dc.Businesses where b.BusinessType.Id == anIdVariable
              select b).ToList();
return result;