C#

• For a reading repository:
  
  
  
  • Non volatile, i.e. they don't change very often. A cache that has to refresh every request is no use.
    
  • Large. You still have to check whether there's been a change (usually involves adding more to the data source and a small amount of overhead), so the cached data needs to be large enough that the request for 'has it changed' isn't as much effort as just getting the data would be. Note that this generally means that you can't cache at the single entity (i.e. one row) level, as finding out if a row changed is as heavy as just returning the row, unless the row is very large and bandwidth to the database is an issue. (If you don't care if your information is out of date, you don't need to ask the DB and you can ignore this. But usually you do care.)
    
  
  
• For a writable repository:
  
  
  
  • Not shared between processes, or not synchronisation-critical. There's no point having a write-back cache if you have to invalidate it immediately to synchronise the data with another process. However, if you are updating things frequently, and don't need every change to commit to the database immediately, you can save a lot of database updates.
    
  
  

NickPace wrote:

var someValue = someList.Where(kvp => kvp.Value.Equals(someStringValue)); // which doesn’t work

var someValue = someList.Where(kvp => kvp.Value.Equals(someStringValue)); // which doesn’t work

var someValue = someList.Where(kvp => kvp.Value.Equals(someStringValue)).Select(kvp=>kvp).FirstOrDefault();

IEnumerable result =  someList.Where(y=>y.Key.Contains(searchGUID));<br />

bool onList = someList.Any(kvp => kvp.Value.Equals(someStringValue));

var result = someList.FindAll(delegate(KeyValuePair<Guid, string> q) {return q.Value.Equals(someStringValue);});

List<KeyValuePair<Guid, string>> kvpList = new List<KeyValuePair<Guid, string>>();

private IEnumerable<string> sList;

private void testKVPToList()
{
    for (int i = 1000; i < 2000; i++)
    {
        Guid guid = new Guid("F4168C51-CEB2-4EBC-B6BF-319B339D" + i.ToString());
        kvpList.Add(new KeyValuePair<Guid, string>(guid,(i*5).ToString()));
    }

    sList = from kvp in kvpList select kvp.Value;

    if(sList.Contains("5000")) Console.WriteLine("5000 found");
    if(sList.Contains("9995")) Console.WriteLine("9995 found");
}