Generic parameter for Linq where clause

As I don't have your Model classes I assumed City as a model class

public class City
{
public string Name { get; set; }
public long Population { get; set; }
}

So, the solution to your problem is to use Expression Trees in C#. You could use Reflection also but that takes lot of time.

I'd write an expression tree delegate to fetch the have the value fetched dynamically.

static Funcbool> GetEqualsExp(string nameOfParameter, string valueToCompare)
{
var parameter = Expression.Parameter(typeof(T));
Expression predicate = Expression.Constant(true);
Expression property = Expression.Property(parameter, nameOfParameter);
Expression equal = Expression.Equal(property, Expression.Constant(valueToCompare));
predicate = Expression.AndAlso(predicate, equal);
return Expression.Lambdabool>>(predicate, parameter).Compile();
}

This will give a delegate back to you so that you can utilize this in LINQ queries.

Here is the main method:

static void Main(string[] args)
{
var lst = new List
{
new City{ Name = "Mumbai", Population = 100},
new City{ Name = "Hyderabad", Population = 200}
};
Console.WriteLine("Enter a city name:");
var valueToFetch = Console.ReadLine();
var dynamicBuild = GetEqualsExp("Name", valueToFetch);
var results = lst.Where(dynamicBuild).FirstOrDefault();
Console.Write(string.Format("City searched: {0}, Population: {1}", valueToFetch, results.Population));
Console.Read();
}

For the purpose of the post I've ignored exception handling and misc stuff.

So, give a name that you'd like to search and send that to GetEqualExp.

Note how the GetEqualsExp("name", valuetofetch); is invoked. GetEqualsExp takes first parameter as the propertyName that you'd like to compare against and second is a string which is the value to compare against the property.

You can utilize generic GetEqualsExp() for any other class as well.

Here is the full solution:

namespace JustTests
{
public class City
{
public string Name { get; set; }
public long Population { get; set; }
}
public class Program3
{
static void Main(string[] args)
{
var lst = new List
{
new City{ Name = "Mumbai", Population = 100},
new City{ Name = "Hyderabad", Population = 200}
};
Console.WriteLine("Enter a city name:");
var valueToFetch = Console.ReadLine();
var dynamicBuild = GetEqualsExp("Name", valueToFetch);
var results = lst.Where(dynamicBuild).FirstOrDefault();
Console.Write(string.Format("City searched: {0}, Population: {1}", valueToFetch, results.Population));
Console.Read();
}
static Funcbool> GetEqualsExp(string nameOfParameter, string valueToCompare)
{
var parameter = Expression.Parameter(typeof(T));
Expression predicate = Expression.Constant(true);
Expression property = Expression.Property(parameter, nameOfParameter);
Expression equal = Expression.Equal(property, Expression.Constant(valueToCompare));
predicate = Expression.AndAlso(predicate, equal);
return Expression.Lambdabool>>(predicate, parameter).Compile();
}
}
}

Output:

Try the above program in a console app.

Let me know if you need anything.

5 Replies

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Karthik Chintala8y ago
As I don't have your Model classes I assumed City as a model class
public class City
{
public string Name { get; set; }
public long Population { get; set; }
}
So, the solution to your problem is to use Expression Trees in C#. You could use Reflection also but that takes lot of time.

I'd write an expression tree delegate to fetch the have the value fetched dynamically.
static Funcbool> GetEqualsExp(string nameOfParameter, string valueToCompare)
{
var parameter = Expression.Parameter(typeof(T));
Expression predicate = Expression.Constant(true);
Expression property = Expression.Property(parameter, nameOfParameter);
Expression equal = Expression.Equal(property, Expression.Constant(valueToCompare));
predicate = Expression.AndAlso(predicate, equal);
return Expression.Lambdabool>>(predicate, parameter).Compile();
}
This will give a delegate back to you so that you can utilize this in LINQ queries.

Here is the main method:
static void Main(string[] args)
{
var lst = new List
{
new City{ Name = "Mumbai", Population = 100},
new City{ Name = "Hyderabad", Population = 200}
};
Console.WriteLine("Enter a city name:");
var valueToFetch = Console.ReadLine();
var dynamicBuild = GetEqualsExp("Name", valueToFetch);
var results = lst.Where(dynamicBuild).FirstOrDefault();
Console.Write(string.Format("City searched: {0}, Population: {1}", valueToFetch, results.Population));
Console.Read();
}
For the purpose of the post I've ignored exception handling and misc stuff.

So, give a name that you'd like to search and send that to GetEqualExp.

Note how the GetEqualsExp("name", valuetofetch); is invoked. GetEqualsExp takes first parameter as the propertyName that you'd like to compare against and second is a string which is the value to compare against the property.

You can utilize generic GetEqualsExp() for any other class as well.

Here is the full solution:
namespace JustTests
{
public class City
{
public string Name { get; set; }
public long Population { get; set; }
}
public class Program3
{
static void Main(string[] args)
{
var lst = new List
{
new City{ Name = "Mumbai", Population = 100},
new City{ Name = "Hyderabad", Population = 200}
};
Console.WriteLine("Enter a city name:");
var valueToFetch = Console.ReadLine();
var dynamicBuild = GetEqualsExp("Name", valueToFetch);
var results = lst.Where(dynamicBuild).FirstOrDefault();
Console.Write(string.Format("City searched: {0}, Population: {1}", valueToFetch, results.Population));
Console.Read();
}
static Funcbool> GetEqualsExp(string nameOfParameter, string valueToCompare)
{
var parameter = Expression.Parameter(typeof(T));
Expression predicate = Expression.Constant(true);
Expression property = Expression.Property(parameter, nameOfParameter);
Expression equal = Expression.Equal(property, Expression.Constant(valueToCompare));
predicate = Expression.AndAlso(predicate, equal);
return Expression.Lambdabool>>(predicate, parameter).Compile();
}
}
}
Output:

Try the above program in a console app.

Let me know if you need anything.
0
samba siva8y ago
Actually that is the switch case.
Based on search type like city,address or pincode, i will send the respective x value to the query. I want the select query should be same for all cases.
I want to send the search type and x value dynamically.
0
Rupesh Kahane8y ago
How is it possible because every time your column name is different, so how you are going to ddistingwish that like if it is city / address / country because x is same for all.

Am I right?
0
samba siva8y ago
I want to pass different column names as parameter in where clause dynamically.

string x="Hyderabad";
var query=pr.city==x;
var result=(from pr in db.Properties
join slp in ShortListProp on pr.PropertyId equals slp.PropertyId into gj
from dta in gj.DefaultIfEmpty()
where (query)
select pr)

Here x is the input, i want to pass same input for different columns like pr.City or pr.Address,pr.Country...etc.
0
Rupesh Kahane8y ago
Refer my article for more information
http://www.c-sharpcorner.com/article/how-to-use-joins-group-by-clause-in-entity-framework-with-linq-c-sharp/
0