C# 如何在where子句中使用ComboBox SelectedValue

我尝试了这段代码，但在

where

子句中出现了一个错误

var subComTbl = from subCom in myDb.SubComTbls
where subCom.AuthorityID.ToString()== Authoritycombo.SelectedValue
select subCom;
SubComcombo.Visible = true;
SubComcombo.DataSource = subComTbl;
SubComcombo.DisplayMember = "SubComName";
SubComcombo.ValueMember = "SubComID";

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似乎您应该注意编译器警告

可能的非预期参考比较；要进行价值比较， 将右侧强制转换为键入“string”

SelectedValue

属性属于对象类型。您应该使用

Authoritycombo.SelectedValue.ToString（）

---

==

的两个操作数应为同一类型，因此如果您在左侧使用

subCom.AuthorityID

，则右侧的另一个操作数应与

AuthorityID

的类型相同，或者如果您使用

Authoritycombo.SelectedValue.ToString（）

左侧的另一个操作数应为字符串类型。

您不需要将

SelectedValue

和

subCom.AuthorityID

转换为字符串来共享这些值

SelectedValue

包含与数据源的属性（或列）类型相同类型的值

在您的情况下，我假设

AuthorityID

和

subcompid

属性是整数
因此，在比较之前，您只需在

SelectedValue

中检查null

SubComcombo.Visible = true;
SubComcombo.DataSource = subComTbl;
SubComcombo.DisplayMember = "SubComName";
SubComcombo.ValueMember = "SubComID";

Int32 selectedID = -1; //default value which for sure not in the database
if(Authoritycombo.SelectedValue != null)
    selectedID = (Int32)Authoritycombo.SelectedValue;

var subComTbl = from subCom in myDb.SubComTbls
                where subCom.AuthorityID == selectedID 
                select subCom;

---

Authoritycombo或subCom.AuthorityID为Null请告诉我它是否解决了问题。@MUAl-t是的，请使用==

SubComcombo.Visible = true;
SubComcombo.DataSource = subComTbl;
SubComcombo.DisplayMember = "SubComName";
SubComcombo.ValueMember = "SubComID";

Int32 selectedID = -1; //default value which for sure not in the database
if(Authoritycombo.SelectedValue != null)
    selectedID = (Int32)Authoritycombo.SelectedValue;

var subComTbl = from subCom in myDb.SubComTbls
                where subCom.AuthorityID == selectedID 
                select subCom;