Using a Lambda Expression Over a List in C#

Hemant Srivastava

C#C# articlesLambda Expression List in C#

17 Comments

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Dinesh Gabhane6y ago
Very Good Article
Himani Patel10y ago
thank you so much it really helped me a lot :)
ricardo jr13y ago
Hi Hermant, thanks for the help! But my problem is that I need analyze a given string like "data > "\"17/05/2013\"" or "code <= "\"123456\"", "data" and "code" can be string attributes from a class, and can be from different classes. After analyzing the string I need make a lambda expression that execute the filtering. For example, I have a class public class Element { public string Name; public double Number; public string data; } var items = new List<Element>() { new Element("a", 1000 ,"13-12-2012 10:47:00:000"), new Element("b", 900 ,"13-12-2012 10:48:00:345"), new Element("b", 800 ,"13-12-2012 10:48:00:545"), new Element("d", 700 ,"13-12-2012 11:48:00:545"), new Element("e", 600 ,"13-12-2012 12:48:00:545"), new Element("x", 500.5 ,"13-12-2012 14:48:00:545"), new Element("y", 400 ,"13-12-2012 15:48:00:545"), new Element("z", 300 ,"13-12-2012 16:48:00:545") }; and for filter on Number I do: string s = "Number >= 500.50 && Number < 1000"; var pred = SimpleExpression.PredicateParser<Element>.Parse(s); var f = pred.Compile(); Now q has my filtered list by query: var q = from e in items where f(e) select e; The SimpleExpression.PredicateParser fails when analize "data > "\"17/05/2013\"" because both are strings. My work to do is make a analizer that generate a expression that can be used on var q = from e in items where f(e) select e; f(e) returns bool, so, if I use IComparer and IComparable I only can return int from Compare and CompareTo. The predicate parser that I using is from http://www.codeproject.com/Tips/355513/Invent-your-own-Dynamic-LINQ-parser thanks!
Hemant Srivastava13y ago
Hi Ricardo, In that situation, You need to implement IComparable interface for Person class. Refer to following link http://support.microsoft.com/kb/320727
ricardo jr13y ago
Hi, I trying to do something like list all SSN >= "203456879", for example. My problem is that both types are strings and comparison is different than numeric types. Does someone have a clue?
wen ze13y ago
Thanks for your sharing.
Gaurav Gupta14y ago
Thanks for sharing. I read that...
Hemant Srivastava14y ago
@Gaurav Gupta: How to use lambda expression on a DataTable object, you can see my new article at: http://www.c-sharpcorner.com/UploadFile/0f68f2/querying-a-data-table-using-select-method-and-lambda-express/
Hemant Srivastava14y ago
@shweta garg: Hi Shweta, Wherever we are getting a list of objects after using lambda expression, we could use "Count" property to get the number of occurances ... For example: listPersonsInCity.FindAll(e => (e.Age >= 60)).Count
Hemant Srivastava14y ago
@Gaurav Gupta: ..... Thanks Gaurav.. Yes we can use lambda expression on data table object. I am going to write another article of using lamda expression over Data table...Wait for a few days ....
shweta garg14y ago
Nice explanation Sir....Sir can we count the number of occurrences of each value in the list?
Megha Goyal14y ago
It is easily understandable article. By this we come to know that how to perform search operation on list using lamda expression.Really very impressive
Tina Garg14y ago
Very nice explanation Hemant Sir..It is very useful for me.Thanks
Gaurav Chauhan14y ago
Very useful article.
lavisha rastogi14y ago
nice article.....
Varesh Tuli14y ago
Nice article to understands lambda. Thanks for sharing.
Gaurav Gupta14y ago
Good article for learn about Lambda. But can you show how to use Lambda experession with DataTable object.

Sometimes if you have a set of records in a List, it becomes quite easy to query on a list using a Lamda Expression. This article exemplifies methods for performing several tasks and queries over a list. A sample code is also attached with this article to explain the use of Lambda Expressions.

Suppose we have a "Person" class that has the following members:

class Person  
{  
    public string SSN;  
    public string Name;  
    public string Address;  
    public int Age;  
    public Person(string ssn, string name, string addr, int age)  
    {  
        SSN = ssn;  
        Name = name;  
        Address = addr;  
        Age = age;  
    }  
}

Now we create a list of the Person objects in which we have to perform several operations like finding a person on certain conditions, removing a person's record etc. These types of operations can be easily performed using a "Lambda Expression". We create the list and populate them in the following way:

List<Person> listPersonsInCity = new List<Person>();  
listPersonsInCity.Add(new Person("203456876", "John", "12 Main Street, Newyork, NY", 15));  
listPersonsInCity.Add(new Person("203456877", "SAM", "13 Main Ct, Newyork, NY", 25));  
listPersonsInCity.Add(new Person("203456878", "Elan", "14 Main Street, Newyork, NY", 35));  
listPersonsInCity.Add(new Person("203456879", "Smith", "12 Main Street, Newyork, NY", 45));  
listPersonsInCity.Add(new Person("203456880", "SAM", "345 Main Ave, Dayton, OH", 55));  
listPersonsInCity.Add(new Person("203456881", "Sue", "32 Cranbrook Rd, Newyork, NY", 65));  
listPersonsInCity.Add(new Person("203456882", "Winston", "1208 Alex St, Newyork, NY", 65));  
listPersonsInCity.Add(new Person("203456883", "Mac", "126 Province Ave, Baltimore, NY", 85));  
listPersonsInCity.Add(new Person("203456884", "SAM", "126 Province Ave, Baltimore, NY", 95));

Now we see how we can do various complex operations on the list using a one-line simple Lambda expression.

The following code retrieves the first two persons from the list who are older than 60 years:

Console.WriteLine("\n-----------------------------------------------------------------------------");  
Console.WriteLine("Retrieving Top 2 aged persons from the list who are older than 60 years\n");  
foreach (Person person in listPersonsInCity.FindAll(e => (e.Age >= 60)).Take(2).ToList())  
{  
    Console.WriteLine("Name : " + person.Name + " \t\tAge: " + person.Age);  
}

The following code checks any person's age that is between 13 and 19 years:

Console.WriteLine("\nChecking whether any person is teen-ager or not...");  
if (listPersonsInCity.Any(e => (e.Age >= 13 && e.Age <= 19)))  
{  
    Console.WriteLine("Yes, we have some teen-agers in the list");  
}

The following code checks whether all the people's ages are greater than Ten years or not:

Console.WriteLine("\nCheking whether all the persons are older than 10 years or not...");  
if ( listPersonsInCity.All(e => (e.Age > 10)))  
{  
    Console.WriteLine("Yes, all the persons older than 10 years");  
}

The following code gets the average of all the people's ages:

Console.WriteLine("\nGetting Average of all the person's age...");  
double avgAge = listPersonsInCity.Average(e => e.Age);  
Console.WriteLine("The average of all the person's age is: "+ avgAge);

The following code checks whether a person having the name 'SAM' exists or not:

Console.WriteLine("\nChecking whether a person having name 'SAM' exists or not...");  
if (listPersonsInCity.Exists(e => e.Name == "SAM"))  
{  
    Console.WriteLine("Yes, A person having name  'SAM' exists in our list");  
}

The following code checks at what position a person having the name 'Smith' exists in the list:

Console.WriteLine("\nChecking the index position of a person having name 'Smith' ...");  
int indexForSmith = listPersonsInCity.FindIndex(e => e.Name == "Smith");  
Console.WriteLine("In the list, The index position of a person having name 'Smith' is : " + indexForSmith);

The following code retrieves the oldest person in the list:

Console.WriteLine("\nGetting the name of the most aged person in the list ...");  
Person p = listPersonsInCity.First(m=> m.Age == (listPersonsInCity.Max(e => e.Age)));  
Console.WriteLine("The most aged person in our list is: "+ p.Name +" whose age is: "+ p.Age);

The following code gets the total of all the people's ages:

Console.WriteLine("\nGetting Sum of all the person's age...");  
int sumOfAges = listPersonsInCity.Sum(e => e.Age);  
Console.WriteLine("The sum of all the persons's age = "+ sumOfAges);

The following code skips each person whose age is less than 60:

Console.WriteLine("\nSkipping every person whose age is less than 60 years...");  
foreach (Person pers in listPersonsInCity.SkipWhile(e => e.Age < 60))  
{  
    Console.WriteLine("Name : "+ pers.Name + " \t\tAge: "+ pers.Age);  
}

The following code retrieves all the people until we find a person with a name beginning with any letter other than "S" :

Console.WriteLine("Displaying the persons until we find a person with name starts with other than 'S'");  
foreach (Person pers in listPersonsInCity.TakeWhile(e => e.Name.StartsWith("J")))  
{  
    Console.WriteLine("Name : " + pers.Name + " \t\tAge: " + pers.Age);  
}

The following code checks whether all the people have their SSN or not:

Console.WriteLine("\nChecking all the persons have SSN or not ...");  
if(listPersonsInCity.TrueForAll(e => e.SSN != null))  
{  
    Console.WriteLine("No person is found without SSN");  
}

The following code removes all the people having the name "SAM":

Console.WriteLine("\nRemoving all the persons record from list that have "SAM" name");  
listPersonsInCity.RemoveAll(e => (e.Name == "SAM"));  
if (listPersonsInCity.TrueForAll(e => e.Name != "SAM"))  
{  
    Console.WriteLine("No person is found with 'SAM' name in current list");  
}

The following code searches for the person having "203456876" as their SSN:

Console.WriteLine("\nFinding the person whose SSN = 203456876 in the list");  
Person oPerson = listPersonsInCity.Find(e => (e.SSN == "203456876"));  
Console.WriteLine("The person having SSN '203456876' is : " + oPerson.Name + " \t\tAge: " + oPerson.Age);