如何使用where子句運算式進行物體框架/Linq加入查詢

我有一個物體框架查詢，其中 where 子句根據幾個選擇而變化。
這是我的代碼：

Expression<Func<CtCsGroup, bool>> where_expression1 = x => 1 == 1;
Expression<Func<CtCsGroup, bool>> where_expression2 = x => 1 == 1;
Expression<Func<CtCsGroup, bool>> where_expression3 = x => 1 == 1;


if (selections.SearchCSGroup.NoNull() != "")
{
    where_expression1 = x => x.CsGroup.ToUpper() == selections.SearchCSGroup.ToUpper();
}

if (selections.SearchUserId.NoNull() != "")
{
    where_expression3 = x => x.UsrIdn.ToUpper() == selections.SearchUserId.ToUpper();
}

if (!selections.SearchIncludeRetires)
{
    where_expression2 = x => x.CsRetire != "Y";
}

var groupDTOs = (from g in _infobaseContext.CtCsGroup
                 .Where(where_expression1)
                 .Where(where_expression2)
                 .Where(where_expression3)
                 select new
                 {
                     UsrIdn = g.UsrIdn,
                     CsGroup = g.CsGroup,
                     CsRetire = g.CsRetire,
                     CsGroupName = "test" // n.CsGroupName
                 }).ToList();

我想將連接添加到另一個表。

如何將此添加到上述查詢中？我嘗試了幾種不同的格式，但每當我添加它時，我都會收到錯誤"(where_expression1)"、"(where_expression3)"等。

謝謝。

這里有更多細節......

var groupDTOs = (from g in _infobaseContext.CtCsGroup
                         join n in _infobaseContext.TblCsGroupList
                                    on g.CsGroup equals n.CsGroup
                         select new
                         {
                             UsrIdn = g.UsrIdn,
                             CsGroup = g.CsGroup,
                             CsRetire = g.CsRetire,
                             CsGroupName =  n.CsGroupName
                         }).ToList();

• 這個沒有加入就可以作業 -

var groupDTOs = (from g in _infobaseContext.CtCsGroup
                         .Where(where_expression1)
                         select new
                         {
                             UsrIdn = g.UsrIdn,
                             CsGroup = g.CsGroup,
                             CsRetire = g.CsRetire,
                             CsGroupName =  "test"  //n.CsGroupName
                         }).ToList();

• 但是當我同時添加連接和運算式時 -

var groupDTOs = (from g in _infobaseContext.CtCsGroup
                         join n in _infobaseContext.TblCsGroupList
                                    on g.CsGroup equals n.CsGroup
                         .Where(where_expression1)
                         select new
                         {
                             UsrIdn = g.UsrIdn,
                             CsGroup = g.CsGroup,
                             CsRetire = g.CsRetire,
                             CsGroupName =  n.CsGroupName
                         }).ToList();

• Visual Studio 出現此錯誤：引數 2：無法從 'System.Linq.Expressions<System.Func<IBDataServices.Models.CtCsGroup, bool>>' 轉換為 'System.Func<char,bool>'

uj5u.com熱心網友回復：

通常，我們會努力通過多個步驟組合查詢，而不是嘗試注入多個預先組合的謂詞。您已經在使用Fluent表示法來應用您的謂詞，因此將該程序從基于Query的運算式中分離出來。

您還會注意到比較值已被引數化，而不是讓linq 解釋器決定什么是查詢參考，什么是我們的離散值或標量值。

您尚未發布具體錯誤，但是在評估linq運算式的elections.SearchCSGroup位置可能超出范圍且無法解決。有意的引數化避免了這種混淆。

IQueryable<CtCsGroup> groupQuery = _infobaseContext.CtCsGroup;
if (selections.SearchCSGroup.NoNull() != "")
{
    // Force the comparison against a discrete parameter value
    var selection = selections.SearchCSGroup.ToUpper();
    groupQuery = groupQuery.Where(x => x.CsGroup.ToUpper() == selection);
}

if (selections.SearchUserId.NoNull() != "")
{
    var selection = selections.SearchUserId.ToUpper();
    groupQuery = groupQuery.Where(x => x.UsrIdn.ToUpper() == selection);
}

if (!selections.SearchIncludeRetires)
{
    // NOTE: "Y" will be correctly interpreted as a discrete parameter value
    groupQuery = groupQuery.Where(x => x.CsRetire != "Y");
}

var groupDTOs = (from g in groupQuery 
                 select new
                 {
                     UsrIdn = g.UsrIdn,
                     CsGroup = g.CsGroup,
                     CsRetire = g.CsRetire,
                     CsGroupName = "test" // n.CsGroupName
                 }).ToList();

最后一部分也可以組成一個流利的運算式，而不是一個基于查詢的運算式：

var groupDTOs = groupQuery.Select(g => new
                {
                    UsrIdn = g.UsrIdn,
                    CsGroup = g.CsGroup,
                    CsRetire = g.CsRetire,
                    CsGroupName = "test" // n.CsGroupName
                }).ToList();

這種模式通過首先嘗試通過預編譯的謂詞來幫助您避免這些問題，特別是它消除了使用不正確的型別系結定義謂詞的可能性。

此模式還有助于組合具有條件排序列或方向的運算式。

您發布的最后一個問題是一個紅鯡魚，您混淆了語法，該運算式從未想過作業。

Visual Studio 出現此錯誤：引數 2：無法從 'System.Linq.Expressions<System.Func<IBDataServices.Models.CtCsGroup, bool>>' 轉換為 'System.Func<char,bool>'

Specifically this section:

join n in _infobaseContext.TblCsGroupList
          on g.CsGroup equals n.CsGroup.Where(where_expression1)

You probably see it now that the line break is removed, n.CsGroup is a string, and it itself IEnumerable<char> so it is assuming you want to apply the where_expression1 predicate to the value of n.CsGroup

We can still join onto the composed query using the normal linq join expressions:

var groupDTOs = (from g in groupQuery
                 join n in _infobaseContext.TblCsGroupList
                        on g.CsGroup equals n.CsGroup 
                 select new
                 {
                     UsrIdn = g.UsrIdn,
                     CsGroup = g.CsGroup,
                     CsRetire = g.CsRetire,
                     CsGroupName = n.CsGroupName
                 }).ToList();

If your schema has a navigation property on CtCsGroup called CsGroup that represents the relationship between TblCsGroupList and CtCsGroup then we can use dot notation and the necessary joins will be implicit.

NOTE: This makes broad assumptions about the schema and is only included to show a common structural alternative to manually defining joins

var groupDTOs = (from g in groupQuery
                 select new
                 {
                     UsrIdn = g.UsrIdn,
                     CsGroup = g.CsGroup,
                     CsRetire = g.CsRetire,
                     CsGroupName = g.CsGroup.CsGroupName
                 }).ToList();

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