Syntax Guide: Using LINQ Expressions for Conditions in Rosters

November 21, 2016

Rosters are common survey instruments (ie household roster, assets roster, food consumption roster). Using Language Integrated Query (LINQ) and lambda expressions in the C# language, we are able to code enabling and validation conditions for rosters that can both:

  • Horizontally: Refer to the values of other variables for the current occurrence
  • Vertically: Refer to the values of the same variable for other items in the roster

In this article, we will cover the general syntax, common operators, provide a few examples on how to write enabling and validation conditions for rosters. Additionally, this article also provide links to more resources on LINQ expressions.
 

General Syntax

The general syntax for rosters is:

RosterName.Operator(x=>x.varQuery1==1...)

 

Item Description
RosterName Name of the roster of interest to query. This is where to find the variables you are interested in.
Operator Query Operator: The action that you want to be performed
x Anonymous variable that captures the queried value. This is just a place holder and can be anything (eg a, b, person)
x.varQuery Variable(s) of interest for the query

The part of the syntax in the parentheses after the operator is called the “lambda expression.” The lambda expression specifies the query criteria for the LINQ expression.
The value that is returned by the statement depends on the operator used. LINQ expressions can also be used to write more complex queries to a look up table.

 

Commonly Used Operators

There are many LINQ query operators that offer a range of query capabilities like filtering, sorting, projection, aggregation, etc. The most useful LINQ operators for Survey Solutions are:

  • All: Checks that all elements of the expression meet the query criteria. This operator returns true/false.
  • Any: Checks that at least one element of the expression meets the query criteria. This operator returns true/false.
  • Count: Counts the number of elements that meet the query criteria. This returns the count.
  • FirstOrDefault: Finds the first element that meets the query criteria. this returns the first element matching the query criteria. In most cases, this will be the first row that matches the search criteria If no element matches the query criteria, then it returns null.

 

Example 1

Assume we want to enable a section only if there are any woman 15-49 years old in the household. For this enabling condition, we will be using the following variable names:

  • Age is the name of the variable recording the age of the household member
  • Gender is the name of the variable recording the gender of the household member. Gender will have a value of 2 if the person is a woman (Gender==2)
  • hhroster is the name of the roster containing the age and gender questions

Using this information, we would code the enabling condition to be like this:

/* Enable only if there are any women 15-49 yo in the household */
hhroster.Any(x=>x.Gender==2 && x.Age>=15 && x.Age<=49)

In words, the expression above says: in the hhroster roster, look to see if there is any household member who is a woman and is between the age of 15 and 49 years old. Since we used the operator Any, this expression will return true if there are any woman 15-49 years old in the household. It will return false if there are none. If instead we wanted to count the number of women 15-49 years old in the household, then we would use the Count operator.

 

Example 2

We want to check that the household contains one and only one household head. This is a common validation in many household surveys. For this approach, we would like to flag an error in the following cases:

  1. There is more than one household head
  2. There is no household head

We will be using the following variable names:

  • relat is the name of the variable name of the question recording the relationship to the household head. relat will have a value of 1 if the person is the household head (relat==1)
  • hhroster is the name of the roster containing the questions about the household member

Since Designer allows for more than one validation conditions for each question, we will write a separate validation condition for each case so that we can provide more detailed error messages to the interviewers.
 

Case 1. There is more than one household head

For this case, we want to check that another person has not been listed as the household head if a household member has been identified as the household head. We will use the conditional expression operator to code this validation. It would be coded like:

 // Is this person the household head?
relat==1 ?

    // If household head, check that there is only one household head listed
    (hhroster.Count(x=>x.relat==1)==1) 

    // If not a household head, then do not do anything
    : true

hhroster.Count(x=>x.relat==1) will return the number of household heads currently listed in the roster. Because we used a conditional expression, relat==1? makes it so that it will only check that there is only one head if the current person was recorded to be a household head. If the person is not listed as a household head, then no error will be flagged.

 

Case 2. There is no household head

For this case, we only want to run this check after question about the relationship to head has been answered for everyone because this validation will trigger for every person until the household head is listed. Similarly, we will use the conditional expression operator to code this validation. It would be coded like this:

// Has relationship to head been answered for all household members?
hhroster.All(x=>x.relat!=null) ?

    //  If all information has been filled out, check for a head was listed
    !(hhroster.Count(x=>x.relat==1)==0)

    // If all information has not been filled, do not do anything
    : true

hhroster.All(x=>x.relat!=null) checks that relationship to head of household has been filled out for every member. If that is true, then !(hhroster.Count(x=>x.relat==1)==0) will count the number of members that are listed as a household head and check that the number of household heads listed is not equal to 0. If not all the relationship to household head has not been filled out for every member, then no error will be flagged.

If you would like to learn more about and test out these two validation conditions, you can refer to this question in the Household Roster public questionnaire.Case 1 is validation condition 1 in the question. Case 2 is validation condition 5 in the question. You can use the Tester application to test the questionnaire.
 

More resources on LINQ

For more information to write more complex LINQ queries in C#: