SelectMany Operator belong to Projection Operators category. It is used to project each element of a sequence to an IEnumerable<T> and flattens the resulting sequences into one sequence.
For eg.. Consider a Student class having a List<string> Subjects Property. Subjects property in this class is a collection of strings.
Example 1: Projects all subject strings of a given a student to an IEnumerable<string>.
In this example since we have 4 students, there will be 4
IEnumerable<string> sequences, which are then flattened to form a
single sequence i.e a single IEnumerable<string> sequence.
IEnumerable<string> allSubjects = Student.GetAllStudetns().SelectMany(s => s.Subjects);
foreach (string subject in allSubjects)
{
Console.WriteLine(subject);
}
Example 2: Rewrite Example1 using SQL like syntax. When using SQL like syntax style, we don't use SelectMany, instead we will have an additional from clause, which will get it's data from the results of the first from clause.
IEnumerable<string> allSubjects = from student in Student.GetAllStudetns()
from subject in student.Subjects
select subject;
foreach (string subject in allSubjects)
{
Console.WriteLine(subject);
}
Example 3: Projects each string to an IEnumerable<char>.
In this example since we have 2 strings, there will be 2
IEnumerable<char> sequences, which are then flattened to form a
single sequence i.e a single IEnumerable<char> sequence.
string[] stringArray =
{
"ABCDEFGHIJKLMNOPQRSTUVWXYZ",
"0123456789"
};
IEnumerable<char> result = stringArray.SelectMany(s => s);
foreach (char c in result)
{
Console.WriteLine(c);
}
Example 4: Rewrite Example3 using SQL like syntax.
string[] stringArray =
{
"ABCDEFGHIJKLMNOPQRSTUVWXYZ",
"0123456789"
};
IEnumerable<char> result = from s in stringArray
from c in s
select c;
foreach (char c in result)
{
Console.WriteLine(c);
}
Example 5: Selects only the distinct subjects
IEnumerable<string> allSubjects = Student.GetAllStudetns()
.SelectMany(s => s.Subjects).Distinct();
foreach (string subject in allSubjects)
{
Console.WriteLine(subject);
}
Another Eg. To Print all the ProductNames of a given Category in Comma separated list.
string ProductNames = db.Categories.Where(x=> x.CategoryID ==1)
.SelectMany(x=>x.Products.Select(p=> p.ProductName)).ToArray()
.Aggregate( (a,b) => a + “, “ + b);
Example 6: Rewrite Example 5 using SQL like syntax.
IEnumerable<string> allSubjects = (from student in Student.GetAllStudetns()
from subject in student.Subjects
select subject).Distinct();
foreach (string subject in allSubjects)
{
Console.WriteLine(subject);
}
Example 7: Selects student name along with all the subjects
var result = Student.GetAllStudetns().SelectMany(s => s.Subjects, (student, subject) =>
new { StudentName = student.Name, Subject = subject });
foreach (var v in result)
{
Console.WriteLine(v.StudentName + " - " + v.Subject);
}
Example 8: Rewrite Example 7 using SQL like syntax.
var result = from student in Student.GetAllStudetns()
from subject in student.Subjects
select new { StudnetName = student.Name, Subject = subject };
foreach (var v in result)
{
Console.WriteLine(v.StudnetName + " - " + v.Subject);
}
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