C# – how do i build dynamic linq queries at runtime using strings

Just stumbled into this oldie...

To do this without the dynamic LINQ library, you just need the code as below. This covers most common scenarios including nested properties.

To get it working with IEnumerable<T> you could add some wrapper methods that go via AsQueryable - but the code below is the core Expression logic needed.

public static IOrderedQueryable<T> OrderBy<T>(
    this IQueryable<T> source, 
    string property)
{
    return ApplyOrder<T>(source, property, "OrderBy");
}

public static IOrderedQueryable<T> OrderByDescending<T>(
    this IQueryable<T> source, 
    string property)
{
    return ApplyOrder<T>(source, property, "OrderByDescending");
}

public static IOrderedQueryable<T> ThenBy<T>(
    this IOrderedQueryable<T> source, 
    string property)
{
    return ApplyOrder<T>(source, property, "ThenBy");
}

public static IOrderedQueryable<T> ThenByDescending<T>(
    this IOrderedQueryable<T> source, 
    string property)
{
    return ApplyOrder<T>(source, property, "ThenByDescending");
}

static IOrderedQueryable<T> ApplyOrder<T>(
    IQueryable<T> source, 
    string property, 
    string methodName) 
{
    string[] props = property.Split('.');
    Type type = typeof(T);
    ParameterExpression arg = Expression.Parameter(type, "x");
    Expression expr = arg;
    foreach(string prop in props) {
        // use reflection (not ComponentModel) to mirror LINQ
        PropertyInfo pi = type.GetProperty(prop);
        expr = Expression.Property(expr, pi);
        type = pi.PropertyType;
    }
    Type delegateType = typeof(Func<,>).MakeGenericType(typeof(T), type);
    LambdaExpression lambda = Expression.Lambda(delegateType, expr, arg);

    object result = typeof(Queryable).GetMethods().Single(
            method => method.Name == methodName
                    && method.IsGenericMethodDefinition
                    && method.GetGenericArguments().Length == 2
                    && method.GetParameters().Length == 2)
            .MakeGenericMethod(typeof(T), type)
            .Invoke(null, new object[] {source, lambda});
    return (IOrderedQueryable<T>)result;
}

Edit: it gets more fun if you want to mix that with dynamic - although note that dynamic only applies to LINQ-to-Objects (expression-trees for ORMs etc can't really represent dynamic queries - MemberExpression doesn't support it). But here's a way to do it with LINQ-to-Objects. Note that the choice of Hashtable is due to favorable locking semantics:

using Microsoft.CSharp.RuntimeBinder;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Dynamic;
using System.Linq;
using System.Runtime.CompilerServices;
static class Program
{
    private static class AccessorCache
    {
        private static readonly Hashtable accessors = new Hashtable();

        private static readonly Hashtable callSites = new Hashtable();

        private static CallSite<Func<CallSite, object, object>> GetCallSiteLocked(
            string name) 
        {
            var callSite = (CallSite<Func<CallSite, object, object>>)callSites[name];
            if(callSite == null)
            {
                callSites[name] = callSite = CallSite<Func<CallSite, object, object>>
                    .Create(Binder.GetMember(
                                CSharpBinderFlags.None, 
                                name, 
                                typeof(AccessorCache),
                                new CSharpArgumentInfo[] { 
                                    CSharpArgumentInfo.Create(
                                        CSharpArgumentInfoFlags.None, 
                                        null) 
                                }));
            }
            return callSite;
        }

        internal static Func<dynamic,object> GetAccessor(string name)
        {
            Func<dynamic, object> accessor = (Func<dynamic, object>)accessors[name];
            if (accessor == null)
            {
                lock (accessors )
                {
                    accessor = (Func<dynamic, object>)accessors[name];
                    if (accessor == null)
                    {
                        if(name.IndexOf('.') >= 0) {
                            string[] props = name.Split('.');
                            CallSite<Func<CallSite, object, object>>[] arr 
                                = Array.ConvertAll(props, GetCallSiteLocked);
                            accessor = target =>
                            {
                                object val = (object)target;
                                for (int i = 0; i < arr.Length; i++)
                                {
                                    var cs = arr[i];
                                    val = cs.Target(cs, val);
                                }
                                return val;
                            };
                        } else {
                            var callSite = GetCallSiteLocked(name);
                            accessor = target =>
                            {
                                return callSite.Target(callSite, (object)target);
                            };
                        }
                        accessors[name] = accessor;
                    }
                }
            }
            return accessor;
        }
    }

    public static IOrderedEnumerable<dynamic> OrderBy(
        this IEnumerable<dynamic> source, 
        string property)
    {
        return Enumerable.OrderBy<dynamic, object>(
            source, 
            AccessorCache.GetAccessor(property), 
            Comparer<object>.Default);
    }

    public static IOrderedEnumerable<dynamic> OrderByDescending(
        this IEnumerable<dynamic> source, 
        string property)
    {
        return Enumerable.OrderByDescending<dynamic, object>(
            source, 
            AccessorCache.GetAccessor(property), 
            Comparer<object>.Default);
    }

    public static IOrderedEnumerable<dynamic> ThenBy(
        this IOrderedEnumerable<dynamic> source, 
        string property)
    {
        return Enumerable.ThenBy<dynamic, object>(
            source, 
            AccessorCache.GetAccessor(property), 
            Comparer<object>.Default);
    }

    public static IOrderedEnumerable<dynamic> ThenByDescending(
        this IOrderedEnumerable<dynamic> source, 
        string property)
    {
        return Enumerable.ThenByDescending<dynamic, object>(
            source, 
            AccessorCache.GetAccessor(property), 
            Comparer<object>.Default);
    }

    static void Main()
    {
        dynamic a = new ExpandoObject(), 
                b = new ExpandoObject(), 
                c = new ExpandoObject();
        a.X = "abc";
        b.X = "ghi";
        c.X = "def";
        dynamic[] data = new[] { 
            new { Y = a },
            new { Y = b }, 
            new { Y = c } 
        };

        var ordered = data.OrderByDescending("Y.X").ToArray();
        foreach (var obj in ordered)
        {
            Console.WriteLine(obj.Y.X);
        }
    }
}

Contrary to the answers here, you DON'T need to worry about encoding if the bytes don't need to be interpreted!

Like you mentioned, your goal is, simply, to "get what bytes the string has been stored in".
(And, of course, to be able to re-construct the string from the bytes.)

For those goals, I honestly do not understand why people keep telling you that you need the encodings. You certainly do NOT need to worry about encodings for this.

Just do this instead:

static byte[] GetBytes(string str)
{
    byte[] bytes = new byte[str.Length * sizeof(char)];
    System.Buffer.BlockCopy(str.ToCharArray(), 0, bytes, 0, bytes.Length);
    return bytes;
}

// Do NOT use on arbitrary bytes; only use on GetBytes's output on the SAME system
static string GetString(byte[] bytes)
{
    char[] chars = new char[bytes.Length / sizeof(char)];
    System.Buffer.BlockCopy(bytes, 0, chars, 0, bytes.Length);
    return new string(chars);
}

As long as your program (or other programs) don't try to interpret the bytes somehow, which you obviously didn't mention you intend to do, then there is nothing wrong with this approach! Worrying about encodings just makes your life more complicated for no real reason.

Additional benefit to this approach: It doesn't matter if the string contains invalid characters, because you can still get the data and reconstruct the original string anyway!

It will be encoded and decoded just the same, because you are just looking at the bytes.

If you used a specific encoding, though, it would've given you trouble with encoding/decoding invalid characters.