foreach(var currentJob in ttJobHead)
{
    foreach (var hdCase in Db.HDCase.Where(hdCase => hdCase.HDCaseNum == currentJob.HDCaseNum))
    {
        currentJob.ProjectID = hdCase.ProjectID;
    }
}

foreach(var t1Row in Table1)
{
    foreach (var t2Row in Table2.Where(r => r.Property1 == t1Row.Property1))
    {
        t1Row.Property2 = t2Row.Poperty2;
    }
}

var currentJob = ttJobHead.Single();
foreach(var hdCase in Db.HDCase.Where(hCase => hCase.HDCaseNum == currentJob.HDCaseNum))
{
    hdCase.ProjectID = currentJob.ProjectID;
}

foreach(var currentJob in ttJobHead)
{
    var hdCase = Db.HDCase.Where(hdCase => hdCase.HDCaseNum == currentJob.HDCaseNum).Last();
    if (hdCase != null)
    {
        currentJob.ProjectID = hdCase.ProjectID;
    }
}