LINQ WHERE语句/忽略条件 [英] LINQ WHERE statement/ignore conditions

问题描述

如果参数为null或为空，我需要忽略WHERE语句中的部分或全部条件 F.E:

I need to ignore some or all conditions in WHERE statement if parameter is null or empty F.E:

我有简单的LINQ查询

I have simple LINQ query

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

如果param1为null或emty，如何忽略x.p == param1?

How can I ignore x.p == param1 if param1 is null or emty?

尝试过

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;

但是得到了

Object reference not set to an instance of an object.

如果第二个组合框的项目为null.怎么了?

In case if second combobox's item is null. What's wrong?

推荐答案

您可以将其添加为条件:

You can add it as a condition:

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

如果param1为null或为空，则条件将始终为true，从而有效地完全忽略" where条件.

If param1 is null or empty, the condition will always be true, which effectively "ignores" the where conditions entirely.

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