How to Automatically Trim Char(N) Values in Entity Framework Using Interceptors?

Automatically Trimming Char(N) Values with Entity Framework Interceptors

In this scenario, you're encountering a challenge where text values stored as char(N) fields in an external database need to be automatically trimmed when retrieved in Entity Framework (EF).

Fortunately, EF 6.1 provides a solution using Interceptors. As explained by Rowan Miller, a program manager for Entity Framework at Microsoft, Interceptors can be used to automatically trim trailing white spaces from string properties in your models.

To implement this solution, follow these steps:

1. Define an StringTrimmerInterceptor class as follows:

// Omitted for brevity (see full code provided below)

2. Create a MyConfiguration class to register the interceptor with EF:

// Omitted for brevity (see full code provided below)

By adding this interceptor and configuration class to your project, EF will automatically trim string values retrieved from char(N) fields. The trimming will occur in the database, ensuring optimal performance.

using System.Data.Entity.Core.Common.CommandTrees;
using System.Data.Entity.Core.Common.CommandTrees.ExpressionBuilder;
using System.Data.Entity.Core.Metadata.Edm;
using System.Data.Entity.Infrastructure.Interception;
using System.Linq;

namespace FixedLengthDemo
{
    public class StringTrimmerInterceptor : IDbCommandTreeInterceptor
    {
        public void TreeCreated(DbCommandTreeInterceptionContext interceptionContext)
        {
            if (interceptionContext.OriginalResult.DataSpace == DataSpace.SSpace)
            {
                var queryCommand = interceptionContext.Result as DbQueryCommandTree;
                if (queryCommand != null)
                {
                    var newQuery = queryCommand.Query.Accept(new StringTrimmerQueryVisitor());
                    interceptionContext.Result = new DbQueryCommandTree(
                        queryCommand.MetadataWorkspace,
                        queryCommand.DataSpace,
                        newQuery);
                }
            }
        }

        private class StringTrimmerQueryVisitor : DefaultExpressionVisitor
        {
            private static readonly string[] _typesToTrim = { "nvarchar", "varchar", "char", "nchar" };

            public override DbExpression Visit(DbNewInstanceExpression expression)
            {
                var arguments = expression.Arguments.Select(a =>
                {
                    var propertyArg = a as DbPropertyExpression;
                    if (propertyArg != null && _typesToTrim.Contains(propertyArg.Property.TypeUsage.EdmType.Name))
                    {
                        return EdmFunctions.Trim(a);
                    }

                    return a;
                });

                return DbExpressionBuilder.New(expression.ResultType, arguments);
            }
        }
    }
}

using System.Data.Entity;

namespace FixedLengthDemo
{
    public class MyConfiguration : DbConfiguration
    {
        public MyConfiguration()
        {
            AddInterceptor(new StringTrimmerInterceptor());
        }
    }
}

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