How can I configure Entity Framework to automatically trim values retrieved for specific columns mapped to char(N) fields?

Question

I'm working with a third-party database in which all text values are stored as char(n). Some of these text values are primary keys, whereas others are just normal human-readable text. For the latter, I want retrieved values to be automatically trimmed.

I know I can add Trim to all of my LINQ to Entities queries, but this is messy, unreliable and unmaintainable. I would like to somehow configure Entity Framework to automatically trim values retrieved from specific columns.

However, I don't know how to do this. I'm using EF's fluent API. The closest thing I've thought of so far is creating additional properties to wrap the real properties with Trim method calls, but this is messy and still not very maintainable. I would also prefer for the trimming to occur in the database rather than the application.

Answer 1

Rowan Miller (program manager for Entity Framework at Microsoft) recently posted a good solution to this which uses Interceptors. Admittedly this is only valid in EF 6.1+. His post is about trailing strings in joins, but basically, the solution as applied neatly removes trailing strings from all of the string properties in your models, automatically, without noticeably affecting performance.

Original blog post: Working around trailing blanks issue in string joins

The relevant code is reposted here, but I encourage you to read his blog post. (Also if you use EF, you should read his blog anyway).

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);
            }
        }
    }
}

Rowan continues: "Now that we have an interceptor, we need to tell EF to use it. This is best done via Code-Based Configuration. We can just drop the following class in the same assembly/project as our context and EF will pick it up."

using System.Data.Entity;

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