Lambda expressions in .Net are denoted by the ‘=>’ symbol.

These powerful operators can be used to create sleek, optimized loops that perform much faster.

The usage below condenses the equivalent of at least 20 lines of two loops.

(Sorry kiddos, no time for VB conversion…C# only this time. ;))

Edit (20101229): Lambda rocks! Simply amazing! Greatest ever!

Ex. 1:

            PaymentMethodCollection visiblePaymentMethods = new PaymentMethodCollection();

            DataSet myPaymentMethods = GlobalDataLoads.PaymentMethods();
            List<DataRow> pmids = myPaymentMethods.Tables[0].Select("Visible = 1").ToList();
            List<int> ids = pmids.Select(i => i["PaymentMethodId"]).Cast<int>().ToList(); 
//.Cast<int>().ToList();
            List<PaymentMethod> pm = availablePaymentMethods.FindAll(i => ids.Contains(i.PaymentMethodId));

            availablePaymentMethods.Clear();
            availablePaymentMethods.AddRange(pm);

Ex. 2:

USER user = tol_dc.USERs.Single(u => u.USER_ID == (int)userid);

Ex. 3:

//DataContext tol_dc;
            List<int> courseids = registrationList.Select(i=>i.MODULE_ID).ToList();
            List<MODULE> modulenames = tol_dc.MODULEs.Where(i=>courseids.Contains(i.MODULE_ID)).Cast<MODULE>().ToList();

Ex. 4: (Get the value of data item inside the child repeater of a parent repeater if the UID is known)

//RepeaterCommandEventArgs e
        if ((stringvalueineed==null) || (stringvalueineed=="")) {
        int uid= (int)((System.Data.DataTable)rptCourses.DataSource).Rows[e.Item.ItemIndex]["UID"];
        stringvalueineed= ((System.Data.DataRow[])(((Repeater)rptOne.Items[e.Item.ItemIndex].FindControl("rptTwo")).DataSource)).Single(i => (int)i["UID"] == uid)["COLUMNNAME"].ToString();
        }

Ex. 5a: (filter a list of available products by removing all excluded products from the list, such as those out of stock)

        private bool Load_AvailableProducts()
        {
            string region = Session["region"];

            lbAvailableProducts.Items.Clear();
            LinqData.DataClassesDataContext dc = new LinqData.DataClassesDataContext();

            List<LinqData.GET_PRODUCT_LISTResult> allproducts = (from p in dc.GET_PRODUCT_LIST(region)
                                       select p).ToList();
            List<int> excproducts = (from ListItem m in lbExcludedProducts.Items
                                       select Convert.ToInt32(m.Value)).ToList();

            List <LinqData.GET_PRODUCT_LISTResult> availproducts = allproducts.Where(p => !excproducts .Contains(p.ProductID)).ToList();

            lbAvailableProducts.DataSource = availproducts;

            lbAvailableProducts.DataTextField = "ProductName";
            lbAvailableProducts.DataValueField = "ProductID";
            lbAvailableMarkets.DataBind();
            return true;
        }

Ex. 5b: (same as above.in this version code is smaller and more load is placed on SQL server rather application (IIS). analyze with SQL profiler and you’ll see the difference)

        private bool Load_AvailableProducts()
        {
            string region = Session["Region"];

            lbAvailableProducts.Items.Clear();
            LinqData.DataClassesDataContext dc = new LinqData.DataClassesDataContext();

            List<LinqData.GET_PRODUCT_LISTResult> availproducts = (from i in dc.GET_PRODUCT_LIST(region)
                                       select i).Where(m => !(from ListItem l in lbExcludedProducts.Items
                                       select Convert.ToInt32(l.Value)).ToList().Contains(m.ProductID)).ToList();

            lbAvailableProducts.DataSource = availproducts;

            lbAvailableProducts.DataTextField = "ProductName";
            lbAvailableProducts.DataValueField = "ProductID";
            lbAvailableProducts.DataBind();
            return true;
        }

The lambda one-liner indicated in the snippet below essentially nests at least two “for/foreach” loops. See my related article here for full source.

Ex. 6:

protected void LoadFactories()
    {
        ClearLists();

        if (ddlSegment.SelectedIndex > 0)
        {
            using (AndroneticsDataContext db = new AndroneticsDataContext(
        ConfigurationManager.ConnectionStrings["AndroneticsConnection1"].ConnectionString))
            {
                List<UP_GET_FactoriesResult> allFactories = db.UP_GET_Factories(Convert.ToInt32(ddlRegion.SelectedValue), null).ToList();

                ////this would be approximately how you would approach traditionally
                //foreach (UP_GET_FactoriesResult r in allFactories)
                //{
                //    if (GetUnregisteredRobots(r.FACTORY_ID).Count != 0)
                //    {
                //        //etc, would need another loop here
                //    }
                //}

                //now for the lambda way
                var Factories = allFactories.Where(
                    r => GetUnregisteredRobots(r.FACTORY_ID).Where(
                        ra => !lstDecommissionRobots.Items.Contains(new ListItem(ra.ROBOT_NAME,ra.ROBOT_ID.ToString()))).Count() != 0);

                ddlFactories.DataSource = Factories;
                ddlFactories.DataTextField = "FACOTRY_TITLE";
                ddlFactories.DataValueField = "FACTORY_ID";
                ddlFactories.DataBind();

                ListItem li = new ListItem("", "");
                ddlFactories.Items.Insert(0, li);
            }
        }
    }

    private List<UP_LOAD_COURSE_RobotsResult> GetUnregisteredRobots(int courseid)
    {
        using (AndroneticsDataContext db = new AndroneticsDataContext(
        ConfigurationManager.ConnectionStrings["AndroneticsConnection1"].ConnectionString))
        {
            List<UP_LOAD_COURSE_RobotsResult> allRobots = db.UP_LOAD_COURSE_Robots(courseid).ToList();

            int userID = int.Parse(Request.QueryString["ID"]);
            List<string> excregisteredRobots = (from m in db.UP_GET_REGISTERED_Robots(userID) select m.MODULE_NAME).ToList();

            return allRobots.Where(m => !excregisteredRobots.Contains(m.MODULE_NAME)).ToList();
        }
    }

Ex. 7 (foreach)

//make sure list first
List<mydatatype> data = ReturnSomeData().ToList();

//technically long way but helps understand what is happening
Func<mydatatype, string> f = (a) => a.somepropertyname = "somevalue";
                    data.ForEach(i => f(i));

//or simply

data.ForEach(c => c.somepropertyname = "somevalue";

Ex. 8 Lambda Cast to Custom Object (list of objects from one type to another)

//DAL.StoreProcedures is simply a static class with functions that instantiate and dispose of datacontexts and return object collections from data calls

List<geocode> geocodes = DAL.StoredProcedures.LoadGeoCodes_Zip(market, datestart).Select(g=> new geocode {lat=g.Latitude.ToString(),lng=g.Longitude.ToString()}).Cast<geocode>().ToList();

References:
MSDN, “Lambda Expressions (C# Programming Guide)”, http://msdn.microsoft.com/en-us/library/bb397687.aspx
MSDN C# Developer Center, “101 Linq Samples”, http://msdn.microsoft.com/en-us/vcsharp/aa336746
Fraction of The Blogosphere, https://ronniediaz.com/2010/12/21/lambda_functions_in_-net/
StackOverflow, http://stackoverflow.com/questions/1909268/convert-a-list-of-objects-from-one-type-to-another-using-lambda-expression