How to Conditionally Apply a Linq Operator

How can I conditionally apply a Linq operator?

if you want to only filter if certain criteria is passed, do something like this

var logs = from log in context.Logs
           select log;

if (filterBySeverity)
    logs = logs.Where(p => p.Severity == severity);

if (filterByUser)
    logs = logs.Where(p => p.User == user);

Doing so this way will allow your Expression tree to be exactly what you want. That way the SQL created will be exactly what you need and nothing less.

Linq: adding conditions to the where clause conditionally

If you do not call ToList() and your final mapping to the DTO type, you can add Where clauses as you go, and build the results at the end:

var query = from u in DataContext.Users
   where u.Division == strUserDiv 
   && u.Age > 18
   && u.Height > strHeightinFeet
   select u;

if (useAge)
   query = query.Where(u => u.Age > age);

if (useHeight)
   query = query.Where(u => u.Height > strHeightinFeet);

// Build the results at the end
var results = query.Select(u => new DTO_UserMaster
   {
     Prop1 = u.Name,
   }).ToList();

This will still only result in a single call to the database, which will be effectively just as efficient as writing the query in one pass.

Conditional Where clause in LINQ

Try this:-

First select the data:-

var r = from t in TblFamilie
select new
{
    t.ID,
    t.ParentID,
    t.Name,
    t.CurDate
};

Then you can filter based on condition:-

   if (sName!="")
        r = r.Where(x => x.Name == sName);

how to put conditional WHERE clause within LINQ query

Just move the condition into the lambda:

var g = mast.Where(w => (condition
    ? (w.Prop1.ToLower() != "default" || w.Prop2.ToLower() != "data")
    : true)
).ToList();

Linq to Entities conditional WHERE

You can use || operator in the query so if first condition is true, the second will not be evaluated and if first is false that is not equal to 0 second will be evaluated:

where lUserId ==0 || (a.PMId == lUserId || a.TLId == lInUserId)

Nested conditional operator in a LINQ query

You can chain many ?: as follows:

var radData = (from stop in dbContext.stop_details
               join del in dbContext.stop_event on stop.id equals del.stop_id into Inners
               from sd in Inners.DefaultIfEmpty()
               where stop.ship_date == startDate && 
                     stop.cust_ref_5_terminalID == "HEND"

               let value = ((int?)sd.ontime_performance)
               select new
               {
                   shipDate = stop.ship_date,
                   custRef = stop.cust_ref_5_terminalID,
                   name = stop.customer.customer_name,
                   ontime = (int?)sd.ontime_performance,
                   OTP = value == null ? "Open" :
                         value < 1 ? "On time" :
                         value == 1 ? "One Day Late" : 
                         value == 2 ? "Two Days Late" : "Three or more days late"
               }).ToList();

Also you can store the field in a variable so you don't need to cast it each time: let value = ((int?)sd.ontime_performance)

BTW - the field being int? you can change the value < 1 to value == 0. Consistent with the other conditions and less confusing

C# how to use ternary conditional inside LINQ Select

It sounds like this should work for you

.Select(x => {
    if (x.get_Parameter(BuiltInParameter.ALL_MODEL_MARK) is Parameter _param)
        return _param.AsString();
    return x.Name;
})

Replacing conditional operator in the linq select with dynamic property

I would suggest to use LINQKit. It needs just configuring DbContextOptions:

builder
    .UseSqlServer(connectionString)
    .WithExpressionExpanding(); // enabling LINQKit extension

Define helper class:

public static class LanguageExtensions
{
    public static Expression<Func<T, string>> NameGetter<T>(string language) 
    {
        var param = Expression.Parameter(typeof(T), "e");

        // simple realization
        var propName = "Name_" + language;

        var body = Expression.PropertyOrField(param, propName);

        return Expression.Lambda<Func<T, string>>(body, param);
    }
}

Then you can use helper in the following way:

public List<ClientDto> GetClients(string locale)
{
    var q = from c in _dbContext.Clients
    select new ClientDto
    {
        Id = c.Id,
        Name = LanguageExtensions.NameGetter<Client>(language).Invoke(e)
    };
    return q.ToList()
}

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