Sharepoint Fields and update / retrive Values

Lookup Field

Field Class: SPFieldLookup
Field Value Class: SPFieldLookupValue

Populating Information:

item["FieldName"] = new SPFieldLookupValue("Title"); // SharePoint will do the lookup as long as the LookupValue's are unique
item.Update();
or
item["FieldName"] = new SPFieldLookupValue(1, "Title");
item.Update();

Retrieving Information:

SPFieldLookupValue itemValue = item["FieldName"] as SPFieldLookupValue;
int id = itemValue.LookupId;
string value = itemValue.LookupValue;

Multiple Lookup Field

Field Class: SPFieldLookup
Field Value Class: SPFieldLookupValueCollection

Populating Information:

SPFieldLookupValueCollection itemValues = SPFieldLookupValueCollection();
itemValues.Add(new SPFieldLookupValue(1, "Title"));
item["FieldName"] = itemValues;
item.Update();

Retrieving Information:

SPFieldLookupValueCollection itemValues = item["FieldName"] as SPFieldLookupValueCollection;
foreach (SPFieldLookupValue itemValue in itemValues)
{
   int id = itemValue.LookupId;
   string value = itemValue.LookupValue;
}

User Field

Field Class: SPFieldUser
Field Value Class: SPFieldUserValue

Populating Information:

web.EnsureUser(@"domain\username");
SPUser user = web.AllUsers[@"domain\username"];
item["FieldName"] = user;
item.Update();

Retrieving Information:

string currentValue = item["FieldName"].ToString();
SPFieldUser userField = list.Fields.GetFieldByInternalName("FieldName");
SPFieldUserValue itemValue = (SPFieldUserValue)userField.GetFieldValue(currentValue);
SPUser user = itemValue.User;

URL Field

Field Class: SPFieldUrl
Field Value Class: SPFieldUrlValue

Populating Information:

SPFieldUrlValue urlValue = new SPFieldUrlValue();
urlValue.Url = "http://www.google.com";
urlValue.Description = "Google";
item["FieldName"] = urlValue;
item.Update();

Retrieving Information:

SPFieldUrlValue urlValue = new SPFieldUrlValue(item["FieldName"].ToString());
string url = urlValue.Url;
string description = urlValue.Description;

Multiple Choice Field

Field Class: SPFieldMultiChoice
Field Value Class: SPFieldMultiChoiceValue

Populating Information:

SPFieldMultiChoiceValue itemValue = new SPFieldMultiChoiceValue();
itemValue.Add("Choice 1");
itemValue.Add("Choice 2");
itemValue.Add("Choice 3");
item["FieldName"] = itemValue;
item.Update();

Retrieving Information:

SPFieldMultiChoiceValue itemValue = new SPFieldMultiChoiceValue(item["FieldName"].ToString());
foreach (string choice in itemValue)
{
   // value is in choice
}