C# Expression tree for string interpolation

Question

I have a program that reads a json file with some property names of a certain class. The values of the configured property names should compose up a key.

Lets take an example:

Class:

class SomeClass
{
    public string PropertyOne { get; set; }
    public string PropertyTwo { get; set; }
    public string PropertyThree { get; set; }
    public string PropertyFour { get; set; }
}

var someClass = new SomeClass
            {
                PropertyOne = "Value1",
                PropertyTwo = "Value2",
                PropertyThree = "Value3",
                PropertyFour = "Value4",
            };

Configuration file:

{
   "Properties": ["PropertyOne", "PropertyTwo"]
}

If I knew the properties at compile time I would have create a lambda like:

Func<SomeClass, string> keyFactory = x => $"{x.PropertyOne}|{x.PropertoTwo}"

Is there a way to compile such a lambda using expressions? Or any other suggestions maybe?

static Expression<Func<T, string>> CreateExpression<T>(string[] props) { var par = Expression.Parameter(typeof(T)); return Expression.Lambda<Func<T, string>>(Expression.Call(typeof(string).GetMethod("Join", new Type[] { typeof(string), typeof(object[]) }), Expression.Constant("|"), Expression.NewArrayInit(typeof(object), props.Select(prop => Expression.Property(par, prop)).ToArray())), par); } ... not working in dotnetfidle since they blocking dynamic lambdas and only creates key as "Prop1|Prop2|Prop3|...|PropN" — Selvin
– Selvin, Commented Sep 7, 2022 at 12:51
so it works as Func<SomeClass, string> keyFactory = x => string.Join("|", x.PropertyOne, x.PropertoTwo); — Selvin
– Selvin, Commented Sep 7, 2022 at 12:58

Svyatoslav Danyliv · Accepted Answer · 2022-09-07 13:32:22Z

1

In Expression Tree, string interpolation is converted to string.Format. Analogue of your sample will be:

Func<SomeClass, string> keyFactory = 
   x => string.Format("{0}|{1}", x.PropertyOne, x.PropertoTwo);

The following function created such delegate dynamically:

private static MethodInfo _fromatMethodInfo = typeof(string).GetMethod(nameof(string.Format), new Type[] { typeof(string), typeof(object[]) });

public static Func<T, string> GenerateKeyFactory<T>(IEnumerable<string> propertyNames)
{
    var entityParam = Expression.Parameter(typeof(T), "e");

    var args = propertyNames.Select(p => (Expression)Expression.PropertyOrField(entityParam, p))
            .ToList();

    var formatStr = string.Join('|', args.Select((_, idx) => $"{{{idx}}}"));

    var argsParam = Expression.NewArrayInit(typeof(object), args);

    var body = Expression.Call(_fromatMethodInfo, Expression.Constant(formatStr), argsParam);
    var lambda = Expression.Lambda<Func<T, string>>(body, entityParam);
    var compiled = lambda.Compile();

    return compiled;
}

Usage:

var keyFactory = GenerateKeyFactory<SomeClass>(new[] { "PropertyOne", "PropertyTwo", "PropertyThree" });

edited Sep 7, 2022 at 13:32

answered Sep 7, 2022 at 12:09

Svyatoslav Danyliv

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6 Comments

Anton Over a year ago

Looks promising! However, I am getting an exception on the Expression.Call; System.InvalidOperationException: 'No method 'Format' on type 'System.String' is compatible with the supplied arguments.' I suppose the Format method is called with Format(params object[]) and not Format(string, params object[])

Svyatoslav Danyliv Over a year ago

Will check. Tested with .net6. Which .net do you use?

Anton Over a year ago

I am using .net 5

Svyatoslav Danyliv Over a year ago

Check updated version.

Anton Over a year ago

Thank you! Your solution is working. I just had to make some minor changes :).

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