How do I create a dynamic Linq query to fill an ASP.NET databound ListView? - asp.net

Question

I am having some trouble figuring out the right way to go about creating a dynamic query that I can use values from DropDownList controls to filter and sort/order the results of a database query to fill a ListView. I am able to hard code individual queries, which works ok, except for the fact that it takes an incredible amount of effort, and is not easily changed.
My code is as follows (using all filters):
queryResult = From product In myEntities.InventoryProducts
Where product.VendorID = ddlFilterVendor.SelectedValue And product.ItemType = ddlItemType.SelectedValue And product.LabelSize = ddlLabelSize.SelectedValue And product.PrintLabel = boolPrint And product.Edited = boolEdited
Order By product.ID Ascending
Select product
Return queryResult
Is there a better method to this? I would like to be able to select the value from each DropDownList and generate a custom WHERE clause, as well as an ORDER BY clause.
Any help would be greatly appreciated, thanks.

Answer 1

I can give you a simple example as to how to to proceed with your idea. I am sure if you look through StackOverflow or search via google you will get code that does a better job of dynamic expression building. The same concept can be used for order by.
void Main()
{
var ops = new List<Ops>
{
new Ops
{
OperandType = typeof(string),
OpType=OpType.Equals,
OperandName = "Name",
ValueToCompare = "MM" // in your case this will be the values from the dropdowns
},
new Ops
{
OperandType = typeof(int),
OpType=OpType.Equals,
OperandName = "ID",
ValueToCompare = 1
},
};
var testClasses = new List<TestClass>
{
new TestClass { ID =1, Name = "MM", Date = new DateTime(2014,12,1)},
new TestClass { ID =2, Name = "BB", Date = new DateTime(2014,12,2)}
};
// this will produce prop => ((prop.Name == "MM") And (prop.ID == 1))
var whereDelegate = ExpressionBuilder.BuildExpressions<TestClass>(ops);
foreach(var item in testClasses.Where(whereDelegate))
{
Console.WriteLine("ID " +item.ID);
Console.WriteLine("Name " +item.Name);
Console.WriteLine("Date" + item.Date);
}
}
// Define other methods and classes here
public enum OpType
{
Equals
}
public class Ops
{
public Type OperandType {get; set;}
public OpType OpType {get; set;}
public string OperandName {get;set;}
public object ValueToCompare {get;set;}
}
public class TestClass
{
public int ID {get;set;}
public string Name {get; set;}
public DateTime Date {get;set;}
}
public class ExpressionBuilder
{
public static Func<T,bool> BuildExpressions<T>( List<Ops> opList)
{
Expression currentExpression= null;
var parameterExpression = Expression.Parameter(typeof(T), "prop");
for(int i =0; i< opList.Count; i++)
{
var op = opList[i];
Expression innerExpression = null;
switch(op.OpType)
{
case OpType.Equals :
{
var propertyExpression = Expression.Property(parameterExpression ,
op.OperandName);
var constExpression = Expression.Constant(op.ValueToCompare);
innerExpression = Expression.Equal(propertyExpression,
constExpression);
break;
}
}
if (i >0)
{
currentExpression = Expression.And(currentExpression, innerExpression);
}
else
{
currentExpression = innerExpression;
}
}
var lambdaExpression = Expression.Lambda<Func<T,bool>>(currentExpression,
new []{parameterExpression });
Console.WriteLine(lambdaExpression);
return lambdaExpression.Compile() ;
}
}