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Discover how to effectively order records in LINQ by ShortName and phone number, with solutions to common pitfalls in C# .
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How to Use External Methods in LINQ OrderBy: A Guide for C# Developers

When working with data in C# , developers often encounter the need to sort records in specific orders. One common request is to utilize LINQ to order data based on multiple criteria, such as by ShortName and phone number. This guide addresses this problem, shares an effective solution, and provides insights into optimizing your approach.

The Problem

You have a collection of records that you want to sort by:

ShortName

Phone Number

The requirement is to first display records with phone numbers and subsequently the ones without them. However, you encounter a System.NotSupportedException because the method you defined for custom ordering cannot be translated into an expression that LINQ to Entities can understand.

Understanding the Exception

The exception states:

'LINQ to Entities does not recognize the method 'System.String Order(DOCR.Domain.Entities.CRM.Contractor)' method, and this method cannot be translated into a store expression.'

This means that any custom external methods cannot be directly used in LINQ queries since they need to be translated into SQL queries that a database can execute.

The Solution

To work around this limitation, we can modify our approach as follows:

Step-by-Step Explanation

Adjust the OrderBy Statement:
Instead of using an external method to sort, incorporate logic directly within the LINQ expression.

Code Implementation:
Below is a refined version of your ordering logic:

[[See Video to Reveal this Text or Code Snippet]]

Breaking it Down:

OrderByDescending(e => e.ShortName.Substring(0, 1)): This orders the collection by the first letter of the ShortName, in descending order.

ThenBy(...): Here, it checks if the phone number is empty (or marked as "---"). If it is, those items are ordered to the end (returning 1), otherwise, they come first (returning 0).

Advantages of This Method

Straightforward: By embedding the necessary logic directly in the query, it avoids the complications of external methods while adhering to LINQ's capabilities.

No Additional Methods Required: This minimizes the number of separate methods, keeping your code concise and easier to maintain.

Optimizing Further

While the provided solution works, if you have additional suggestions, consider the following for optimization:

Caching Phone Numbers: If you anticipate this query being run often, you might explore caching phone numbers to reduce the need for repeatedly querying contact details.

Database Indexing: Ensure that your database fields involved in sorting are properly indexed, which can improve performance during sorting operations.

Conclusion

Sorting records in LINQ using multiple criteria can pose challenges, especially when dealing with custom methods. By adapting your sorting approach and directly incorporating logic into your LINQ statement, you can effectively complete the task without encountering common exceptions.

Feel free to share any additional tips for enhancing LINQ queries or new strategies you've developed in your application!