Using A Parametr (string) in LINQ

Question

4.50/5 (2 votes)

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Hi EveryBody, i'm trying Too use a listBox for Select the Search,
but i can't use the string si in LINQ, i don't know how use it,
i made this code, but don't know how use the si.

C#

var db = new DataClasses1DataContext();
int selectedIndex = listBox1.SelectedIndex;
string si = "";
if (selectedIndex == 0) si = "id" ;
if (selectedIndex == 1) si = "name";
if (selectedIndex == 2) si = "sex";
if (selectedIndex == 3) si = "address";
if (selectedIndex == 4) si = "phone";
if (selectedIndex == 5) si = "web";
var qsearch = db.users.Where(c => c.[i want use si in this place].Contains(textBox7.Text));
dataGridView1.DataSource = qsearch;

Posted 29-Jan-15 9:57am

Namvarii

Updated 29-Jan-15 9:59am

v2

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3 solutions

Solution 2

C#

string searchText = textBox7.Text;
var qsearch = db.users.Where(c => c.id.ToString().Contains(searchText) || c.name).Contains(searchText) || c.sex.ToString.Contains(searchText) ||
c.address.Contains(searchText) || c.phone.ToString().Contains(searchText) ||
c.web.ToString().Contains(searchText));

Posted 29-Jan-15 12:10pm

AG1220

Updated 2-Feb-15 9:11am

v4

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**Zoltán Zörgő** · Accepted Answer · 2015-01-29T11:23:00

Solution 1

Good one!

You need to build dynamic expression. Something like this:

C#

var parameter = Expression.Parameter(typeof(user), "p");
var property = Expression.Property(parameter, si);
var check = Expression.Call(property, typeof(String).GetMethod("Contains"), Expression.Constant(textBox7.Text));
var query = db.users;
var predicate = Expression.Lambda<Func<user, bool>>(check, parameter);
var result = query.Where(predicate);

To build a more complex query, use the approaches from here: http://blog.micic.ch/net/entity-framework-and-expression-queries[^]

Posted 29-Jan-15 11:23am

Zoltán Zörgő

Comments

George Swan 29-Jan-15 18:18pm

I think you may have forgotten to compile the Expression.
var predicate = Expression.Lambda<Func<user, bool>>(check, parameter).Compile();

Zoltán Zörgő 29-Jan-15 18:19pm

It is working even without.

George Swan 29-Jan-15 18:34pm

It's a great solution. I'd have declared an array of delegates and indexed into the required one in the Where statement.
var TestFunctions = new Func<users, bool>[6];
TestFunctions[0] = c=>c.id.Contains(textBox7.Text);
TestFunctions[1] = c=>c.name.Contains(textBox7.Text);
//........................
var qsearch = db.users.Where(testFunctions[selectedIndex]);

Maciej Los 8-Feb-15 12:39pm

It's worth to post as an answer.

Maciej Los 8-Feb-15 12:40pm

+5!

Zoltán Zörgő 8-Feb-15 16:56pm

Thank you.

Namvarii · Accepted Answer · 2015-01-30T00:34:00

with this code we can solve this proble, use switch for solve

C#

var db = new DataClasses1DataContext();
int selectedIndex = listBox1.SelectedIndex;
switch (selectedIndex)
{
   case 0:
     dataGridView1.DataSource = db.users.Where(c => c.id.ToString().Contains(...));
     break;
   ....
 }