.NET Megathread 3.5: await GetGoodPosts()

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > The Cavern of COBOL > .NET Megathread 3.5: await GetGoodPosts()

Admiral Snackbar: Mar 13, 2006; OUR SNEEZE SHIELDS CANNOT REPEL A HUNGER OF THAT MAGNITUDE

I have a question about including an expression in an EF select operation. If I run this code against SQL Server:

code:

var tc = ctx.TestClasses
            .Select(x => new
            {
                FullName = x.GroupName != null && x.DisplayName != null
                           ? $"{x.GroupName.Substring(0, 3).ToUpper()}: {x.DisplayName}"
                           : string.Empty
            })
            .ToList();

much of the work of the lambda is translated into SQL and run server-side:

code:

SELECT CASE
    WHEN ([t].[GroupName] IS NOT NULL) AND ([t].[DisplayName] IS NOT NULL) THEN CAST(1 AS bit)
    ELSE CAST(0 AS bit)
END, UPPER(SUBSTRING([t].[GroupName], 0 + 1, 3)), [t].[DisplayName]
FROM [TestClasses] AS [t]

Now that's great for this one case, but if I want to reuse the contents of the lambda elsewhere, I don't see a way to factor it out of this query but keep it running server-side. I think I should be able to do it with an expression, but doing this:

code:

Expression<Func<TestClass, string>> fullName = x => x.GroupName != null && x.DisplayName != null
                                                  ? $"{x.GroupName.Substring(0, 3).ToUpper()}: {x.DisplayName.Substring(0, 3)}"
                                                  : string.Empty;

var tc = ctx.TestClasses
            .Select(x => new
            {
                FullName = fullName.Compile()(x)
            })
            .ToList();

causes the lambda to run client-side instead:

code:

SELECT [t].[Id], [t].[DisplayName], [t].[GroupName]
FROM [TestClasses] AS [t]

Is there a way to incorporate my expression into the EF expression tree to push the work back to the server?

# ¿ Jul 11, 2023 15:06

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# ¿ May 17, 2024 16:52

Admiral Snackbar: Mar 13, 2006; OUR SNEEZE SHIELDS CANNOT REPEL A HUNGER OF THAT MAGNITUDE

I appreciate everyone's feedback on this. I wanted to see if I could make this work by manipulating expression trees, and I put together a visitor that actually works (at least for anonymous types):

code:

internal class PropertyExpressionVisitor<T> : ExpressionVisitor
{
    private readonly Dictionary<string, Expression> propertyExpressions;

    public PropertyExpressionVisitor(Dictionary<string, Expression> propertyExpressions)
    {
        this.propertyExpressions = propertyExpressions;
    }

    [return: NotNullIfNotNull("node")]
    public override Expression? Visit(Expression? node)
    {
        if (node is not LambdaExpression lambda 
         || lambda.ReturnType != typeof(T)
         || lambda.Body is not NewExpression oldConstructorExp
         || oldConstructorExp.Constructor?.DeclaringType != typeof(T)
         || oldConstructorExp.Arguments == null)
            return base.Visit(node);

        var arguments = new List<Expression>(oldConstructorExp.Arguments);            

        foreach (var pe in propertyExpressions.Keys)
        {
            var membIndex = oldConstructorExp.Members?
                                             .Select((m, i) => new { m, i })
                                             .Where(mi => mi.m.Name == pe)
                                             .SingleOrDefault()?.i;

            if (!membIndex.HasValue)
                continue;

            var propExp = Expression.Invoke(propertyExpressions[pe], lambda.Parameters);
            arguments[membIndex.Value] = propExp;
        }

        var newConstructorExp = Expression.New(oldConstructorExp.Constructor, arguments);
        var newLambda = Expression.Lambda(newConstructorExp, lambda.Parameters);

        return newLambda;
    }
}

That paired with this thing successfully convice EF to do the operations server-side:

code:

public static IQueryable<TResult> SelectWithExpressionProperties<TSource, TResult>(this IQueryable<TSource> source, Expression<Func<TSource, TResult>> selector, params KeyValuePair<string, Expression>[] propertySelectors)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (selector == null) throw new ArgumentNullException(nameof(selector));
    if (propertySelectors == null) throw new ArgumentNullException(nameof(propertySelectors));

    var select = source.Select(selector);
    var visitor = new PropertyExpressionVisitor<TResult>(propertySelectors.ToDictionary(ps => ps.Key, ps => ps.Value));            
    var updatedSelect = select.InterceptWith(visitor);

    return updatedSelect;
}

Calling it looks like this (the anonymous type needs placeholder properties for this to work):

code:

var ctx = new Context();

Expression adder = (TestClass x) => x.Id + 17;

var tc = ctx.TestClasses
            .SelectWithExpressionProperties(x => new
                {
                    Id = 0,
                    FullName = ""
                }, 
                new KeyValuePair<string, Expression>("FullName", ExpressionsHelper.FullName),
                new KeyValuePair<string, Expression>("Id", adder))
            .ToList();

Which generates this SQL:

code:

SELECT [t].[Id] + 17, CASE
    WHEN ([t].[GroupName] IS NOT NULL) AND ([t].[DisplayName] IS NOT NULL) THEN CAST(1 AS bit)
    ELSE CAST(0 AS bit)
END, UPPER(SUBSTRING([t].[GroupName], 0 + 1, 3)), SUBSTRING([t].[DisplayName], 0 + 1, 3)
FROM [TestClasses] AS [t]

Now, whether or not this is actually a good idea is another question...

# ¿ Jul 12, 2023 14:06

Admiral Snackbar: Mar 13, 2006; OUR SNEEZE SHIELDS CANNOT REPEL A HUNGER OF THAT MAGNITUDE

I've been learning how to write analyzers and code fix providers, and I have a question about bundling them in NuGet packages. For example, I have an existing NuGet package that is used by some other projects. I have written some analyzers and fixers to address some common issues that have come up when using the NuGet package. Is there a way to update the existing package to include the new analyzers and have those analyzers automatically run when a client project updates its package version?

# ¿ Apr 3, 2024 21:15

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