LINQ WHERE语句/忽略条件

如果参数为null或空，我需要忽略WHERE语句中的部分或全部条件F.E:

我有一个简单的LINQ查询

var query = from x in context.a
            where x.p == param1 && x.i == param2
            select x;

如果param1为空或空，我如何忽略x.p == param1 ?

编辑

尝试这个

var query = from myLog in myContext.ApsValidationLogs
            where (myLog.systemtype == comboBoxSystemType.SelectedItem.ToString() || string.IsNullOrEmpty(comboBoxSystemType.SelectedItem.ToString()))
              && (myLog.bankid == comboBoxBankId.SelectedItem.ToString() || string.IsNullOrEmpty(comboBoxBankId.SelectedItem.ToString())))
            select myLog;

但是得到

Object reference not set to an instance of an object.

如果第二个组合框的项为空。怎么了?

 var query= from x in context.a 
            where String.IsNullOrEmpty(param1) || (x.p == param1 && x.i == param2)
            select x;

var query = from x in context.a
             where (X.p==param1 || string.IsNullOrEmpty(param1))
             && (X.i==param2 || string.IsNullOrEmpty(param2))
             select x;