SelectWhere method

[JeevanJames]

Sometimes we need to perform a heavy operation in the Where clause to satisfy the predicate and then do the same operation in the Select clause to return the final projection.

For example:

var results = students
    .Where(s => CalculateGrade(s.Marks) == 'A')
    .Select(s => new { Name = s.Name, Grade = CalculateGrade(s.Marks) });

This is possible with the let keyword when using LINQ syntax:

var results = from s in students
    let grade = CalculateGrade(s.Marks)
    where grade == 'A'
    select new { Name = s.Name, Grade = grade };

But with method syntax, it is difficult and we typically end up running the operation multiple times. Once recommendation is to create a projection with the initial object and calculated object and work with that:

var results = students
    .Select(s => new { Student = s, Grade = CalculateGrade(s.Marks) })
    .Where(s => s.Grade == 'A')
    .Select(new { Name = s.Student.Name, Grade = s.Grade });

The SelectWhere method enables this scenario without needing the projection.