Скачать или смотреть How to Use SelectNodes for XML Queries with Null Values in C- and VB.NET

Learn how to correctly implement XPath queries in C- and VB.NET to include null values when selecting nodes in XML documents.
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This video is based on the question https://stackoverflow.com/q/68951708/ asked by the user 'Eric' ( https://stackoverflow.com/u/10444833/ ) and on the answer https://stackoverflow.com/a/68954475/ provided by the user 'dbasnett' ( https://stackoverflow.com/u/66532/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Optimizing XML Queries with SelectNodes: Handling Null Values

When working with XML data, especially in programming languages like C- and VB.NET, you often face challenges in selecting specific nodes based on their values. One common scenario is needing to retrieve nodes that can either have specific numeric values or may be null. In this guide, we will explore how to effectively utilize the SelectNodes method in XML queries to include nodes with null values as well.

The Problem: Including Null Values in Node Selection

Imagine you have an XML structure that contains products, and each product has a node called polispremie_1. You need to fetch all product nodes where polispremie_1 has a value of 0.00, 0, or 0,00, and also include any product nodes where polispremie_1 is null (which may be represented as an empty element in XML).

Here's what your XML might look like:

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

Current XPath Query Limitations

Initially, you might be trying a query like this:

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

However, this query will only return product nodes with specific numeric values and will leave out any nodes where polispremie_1 is null (empty). Adding a check for null values using or .= null isn't effective, as you've experienced.

The Solution: Adapting Your Query

To ensure that your XML queries fetch both the numeric values and the null (or empty) nodes, you can modify your XPath query to account for these conditions.

Updated XPath Query

The solution lies in enhancing your XPath expression to include empty nodes like so:

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

This addition checks for empty strings, thus effectively capturing nodes where polispremie_1 does not contain any value.

In Practice: VB.NET Example

Additionally, if you're working within a VB.NET context, you can apply LINQ to XML for a more structured approach. Here’s how that would look:

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

Breakdown of the VB.NET Example

Data Structure: The XML structure is defined clearly using XElement.

LINQ Query: The Where clause effectively filters the nodes based on:

Numeric checks for zero values.

A condition for empty strings (indicating null or missing values).

Conclusion

By utilizing effective XPath expressions and Linq to XML queries, you can efficiently retrieve nodes with either numeric values or null conditions, ensuring you have robust data handling in your XML processing solutions. This ensures that your applications can manage data in a flexible, accurate manner, enhancing usability and data integrity.

By implementing the changes specified here, you'll not only solve the immediate issue but also fortify your approach to querying XML data in your future programming ventures.