Comparing the Search String with the Array in WPF

Question

Hi,
This is the first time i am using WPF with LINQ
I am writing the below code to load the listbox

string usageIndicator = AppEnvironment.ToUpper() == "PROD" ? "P" : "T";

using (X12837DataContext dc = new X12837DataContext(_ediConnectionstring))

{

IList<tbl837SubmissionGroupData> submissionGroups = dc.tbl837SubmissionGroupDatas.Where(s => s.UsageIndicator == usageIndicator &&
(s.SelectClaimsByDate == "Y" || s.SelectClaimsNotSent == "Y")).OrderBy(g => g.SubmissionGroupName).ToList();
lstBoxSubmissionGroup.ItemsSource = submissionGroups;
lstBoxSubmissionGroup.DisplayMemberPath = "SubmissionGroupName";
ResponseText = string.Format("{0} submission groups listed.\n", submissionGroups.Count);
}

where tbl837SubmissionGroupData is a class containing all the attributes ,
submissionGroups andsubmissionGroupName are the attributes.

now the data is like submissiongroups have [0],[1],[2],[3]
which internall contains variables...
[0] ==> id, groupname,groupid

so i want to check all the list [0][1][2][3] data to find a string suppose "mat"which is a unique value in the entire list and get its index.

for example i found mat in [2] then i want to return 2 as index

how to do that. i wrote the following code after the above code but no use i am getting nowher could u please suggest a better approach or help me with any link
IEnumerable items = lstBoxSubmissionGroup.Items;
foreach (object obj in items)
{
string val = obj as string;
if ((!string.IsNullOrEmpty(val)) && val.Contains(searchString))
{
int id = lstBoxSubmissionGroup.Items.IndexOf(obj);
}
}