How can i append a Where String in to Linq?

Question

I am trying to make a dynamic where clause and then embed it in the to LINQ query but i don't know how to do it.

StringBuilder whereClause = new StringBuilder();
    if (groupId>=0)
    {
        whereClause.Append("type.GroupID == groupId");
    }
    if (siteId>=0)
    {
        if (whereClause.Equals("type.GroupID == groupId"))
        {
            whereClause.Append("&&");
        }
        whereClause.Append("type.SiteID == siteId");
    }
    if (string.IsNullOrEmpty(typeName))
    {
        if (whereClause.Equals("type.GroupID == groupId&&type.SiteID == siteId"))
        {
            whereClause.Append("&&");
        }
        whereClause.Append("type.ThermometerTypeName == typeName");
    }
var thermoTypes = execore from type in db.TempR_ThermometerType
    where whereClause
    select type).ToList();

Answer 1

You're going around the whole reason LINQ exists!

Try this instead:

var query = from type in db.TempR_ThermometerType
            select type;

if (groupId >= 0)
{
    query = query.Where(type => type.GroupID == groupId);
}
if (siteId >= 0)
{
    query = query.Where(type => type.SiteID == siteId);
}

...

return query.ToList();

Answer 2

if you need simple and logic just use multiple Where clauses:

var query =  db.TempR_ThermometerType.AsQueryable();

if(..)
{
    query.Where(...);
}

if(..)
{
    query.Where(...);
}

....

to generate your dynamic clauses use PredicateBuilder or via Expressoin 's

PredicateBuilder example from link:

IQueryable<Product> SearchProducts (params string[] keywords)
{
  var predicate = PredicateBuilder.False<Product>();

  foreach (string keyword in keywords)
  {
    string temp = keyword;
    predicate = predicate.Or (p => p.Description.Contains (temp));
  }
  return dataContext.Products.Where (predicate);
}

Answer 3

Or you can use boolean logic (and the fact that multiple where statements imply conjunction - have AND between them) and write one query:

from type in db.TempR_ThermometerType
where groupId < 0 || type.GroupId == groupId
where siteId < 0 || type.SiteId == siteId
where typename == "" || type.TheromometerTypeName == typeName
select type;

The query is bigger, but the code is quite concise and the DB server would only have to cache one parametrized query execution plan instead of several.