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Learn how to resolve the "Must Declare the Scalar Variable" error in Dapper when using DynamicParameters. This guide provides a clear explanation and solution to help you efficiently handle this common issue.
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Fixing the "Must Declare the Scalar Variable" Error in Dapper DynamicParameters

When working with SQL Server 2016 and Dapper, a lightweight Object-Relational Mapping (ORM) tool, developers occasionally encounter the error: "Must declare the scalar variable." This error can be frustrating, especially when the code appears to be correctly structured. In this guide, we'll explore what causes this error and how to fix it using Dapper's DynamicParameters.

What Causes the Error?

The "Must declare the scalar variable" error occurs when a parameter expected by the SQL query is not provided or correctly declared. In the context of Dapper, this usually happens due to a mismatch between the parameter names used in the SQL query and those provided through the DynamicParameters object.

Understanding DynamicParameters in Dapper

Dapper’s DynamicParameters class allows for the addition of parameters in a dynamic way, providing flexibility and ease of use. Parameters are usually added using the Add method. For instance:

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Common Mistake

A common mistake that leads to the "Must declare the scalar variable" error is specifying incorrect parameter names or failing to add parameters that the query expects. For example:

SQL Query:

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If the parameters are added as:

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The above code will trigger the error because the @UserName parameter is not declared.

Resolution

Verify Parameter Names: Ensure that the parameter names in your SQL query match exactly with those in the DynamicParameters.

Add Missing Parameters: Cross-check that all parameters required by the query are added to the DynamicParameters.

Example

Correct Usage:

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In this example, both parameters @UserId and @UserName are declared and added correctly, thus preventing the scalar variable declaration error.

Conclusion

The "Must declare the scalar variable" error in Dapper's DynamicParameters primarily stems from unmatched or missing parameters. By carefully ensuring that all required parameters are declared and correctly named, this issue can be effectively resolved. This approach not only eliminates the error but also leads to more efficient and readable code.

If you encounter this error, methodically review your parameter declarations and SQL query syntax to ensure alignment. This will save you time and ensure smooth database operations with Dapper.