Dynamic query with Linq

In this post I am going to discuss about building dynamic query with the LINQ. LINQ to SQL allow user to query data from the database without writing sql queries by writing LINQ queries. LINQ represent each table as one entity and where LINQ queries allows to manipulate data in type safe.

But Static LINQ queries not able to meet all our programming needs. A Dynamic LINQ queries is needed when we need to retrieve a set of records based on different search parameters.

For example - An employee search screen or a general purpose report which needs to execute a different SELECT statement based on a different WHERE as well as Sorting column to sort data.

Dynamic query in Sql server

In SQL there is concept of dynamic queries which allow to write and execute dynamic queries easily. In SQL server we use EXECUTE or sp_executesql to execute dynamic query.
For example:

DECLARE @SQLQuery AS NVARCHAR(500)
SET @SQLQuery = 'SELECT * FROM tblEmployees WHERE EmployeeID = + convert(@EmpID as varchar(10))
EXECUTE(@SQLQuery) 

or 

DECLARE @SQLQuery AS NVARCHAR(500)
DECLARE @ParameterDefinition AS NVARCHAR(100)
DECLARE @EmpID INT
SET @EmpID =100
SET @SQLQuery = 'SELECT * FROM tblEmployees WHERE EmployeeID = @EmpID'
SET @ParameterDefinition =  '@EmpID INT'
EXECUTE sp_executesql @SQLQuery, @ParameterDefinition, @EmpID

What to do when I am using LINQ ?

There are two way to achieve this thing easily
1. Use Dynamic LINQ libarary
2. Use PredicateBuilder

To understand both of the above library consider the below screen shot

I want to search data by entering in above screen. Note here I may left some field blank and some field filled with value.

Use Dynamic LINQ library
Dynamic LINQ library allows build query which are having varying where clause or orderby. To work with the dynamic LINQ library you need to download and install file in your project.
you get the file from this link : Basic Dynamic LINQ C# Sample

so once you install the file you can build query as we can do in pl-sql query.

Following code demonstrate how to build where dynamically

string strWhere = string.Empty;
        string strOrderBy = string.Empty;

        if (!string.IsNullOrEmpty(txtAddress.Text))
            strWhere = "Address.StartsWith(\"" + txtAddress.Text + "\")";  
        if (!string.IsNullOrEmpty(txtEmpId.Text))
        {
            if(!string.IsNullOrEmpty(strWhere ))
                strWhere = " And ";
            strWhere = "Id = " + txtEmpId.Text;
        }
        if (!string.IsNullOrEmpty(txtDesc.Text))
        {
            if (!string.IsNullOrEmpty(strWhere))
                strWhere = " And ";
            strWhere = "Desc.StartsWith(\"" + txtDesc.Text + "\")";
        }
        if (!string.IsNullOrEmpty(txtName.Text))
        {
            if (!string.IsNullOrEmpty(strWhere))
                strWhere = " And ";
            strWhere = "Name.StartsWith(\"" + txtName.Text + "\")";
        }

        EmployeeDataContext edb = new EmployeeDataContext();
        var emp = edb.Employees.Where(strWhere);
        grdEmployee.DataSource = emp.ToList();
        grdEmployee.DataBind();

In above code I am building strWhere dynamically because there may be some criteria no have value where some has.


Predicate Builder
Predicate builder works same as dynamic linq library but the main difference is its allow to write more type safe queries easily.
You can get the detail about predicate builder form here : Dynamically Composing Expression Predicates
Following code shows how you can use PredicateBuilder easily to create dynamic clause easily.

var predicate = PredicateBuilder.True();

        if(!string.IsNullOrEmpty(txtAddress.Text))
            predicate = predicate.And(e1 => e1.Address.Contains(txtAddress.Text));
        if (!string.IsNullOrEmpty(txtEmpId.Text))
            predicate = predicate.And(e1 => e1.Id == Convert.ToInt32(txtEmpId.Text));
        if (!string.IsNullOrEmpty(txtDesc.Text))
            predicate = predicate.And(e1 => e1.Desc.Contains(txtDesc.Text));
        if (!string.IsNullOrEmpty(txtName.Text))
            predicate = predicate.And(e1 => e1.Name.Contains(txtName.Text));

        EmployeeDataContext edb= new EmployeeDataContext();
        var emp = edb.Employees.Where(predicate);
        grdEmployee.DataSource = emp.ToList();
        grdEmployee.DataBind();

So as you see in above code I had created one PredicateBuilder for AND condition and building where clause same way you can build OR clause by using PredicateBuilder.

Difference between both library

• Predicatebuilder allows to build TypeSafe dynamic queries.
• Dynamic LINQ library allows to builder query with the Dynamic ORDER BY clause.