"Where"子句中的动态表达式 - Linq to SQL

Question

我是LINQ的新手,所以我希望这不是一个愚蠢的问题:

我在数据网格中有一个包含大量内容的表格,我希望用户能够通过使用网格上方的一些组合框来过滤网格[如搜索栏]

我创建了一个方法,它接受组合框中的文本,并将其放在"Where"子句中:

    public void find()
    {
        string disName;
        string statusName;


        disName = RMcmbDis.Text; //This Get the first string to filter
        statusName = RMcmbStatus.Text; // this get the second string to filter

//在这里,我收集了我需要的所有数据

        var allCNT = from x in cntDB.releases
                     join dis in cntDB.disciplines on x.discipline equals dis.discipline_id
                     join btch in cntDB.batches on x.batch_num equals btch.batch_id
                     join z in cntDB.status on x.status equals z.status_id

                     select new { dis.discipline_name, x.grade, x.batch_num, btch.batch_name, z.status_description, x.segment_leader, x.ped_leader, x.release_location, x.comments, x.QA_Verdict };

//我在这里进行过滤

        var find = allCNT.Where(a => a.discipline_name == disName && a.status_description == statusName);


        dataGridView1.DataSource = find;
    }

现在我遇到了一个问题:我希望用户能够将其中一个组合框留空,如果他这样做,这意味着他不想过滤该标准.[EG - 组合"RMcmbDis"具有"Math"并且Status组合["RMcmbStatus"]为空,因此网格将仅在所有状态中显示"Math".

我怎么做？谢谢你们... N.

Answer 1

Where()如果你想要的条件是真的,你可以添加条款...

var results = allCNT;

if (!string.IsNullOrEmpty(disName))
    results = result.Where(a => a.discipline_name == disname);

if (!string.IsNullOrEmpty(statusName))
    results = results.Where(a => a.status_description == statusName);

dataGridView1.DataSource = results;

请参阅下面的注释,了解处理大量过滤器的一个选项.另一种选择是使用辅助方法:

T AddFilter<T>(IQueryable<T> results, string filterValue, Expression<Func<T, bool>> predicate)
{
    if(!string.IsNullOrEmpty(filterValue))
        return results.Where(predicate);
    return results;
}

你会用这个:

var results = allCNT;
results = AddFilter(results, disname, a => a.discipline_name == disname);
results = AddFilter(results, statusName, a => a.status_description == statusName);
results = AddFilter(results, whatever, a => a.whatever == whatever);
// ...
dataGridView1.DataSource = results;