prep the field for building the select statement next.

PoweredSoft.DynamicLinq/Constants.cs

@@ -41,6 +41,16 @@ namespace PoweredSoft.DynamicLinq
         Descending
     }
 
+    public enum SelectTypes
+    {
+        Key,
+        Count,
+        LongCount,
+        Sum,
+        Average,
+        ToList
+    }
+
     internal static class Constants
     {
         internal static readonly MethodInfo GroupByMethod = typeof(Queryable).GetMethods().First(t => t.Name == "GroupBy");

PoweredSoft.DynamicLinq/Extensions/QueryableExtensions.cs

@@ -88,5 +88,15 @@ namespace PoweredSoft.DynamicLinq
 
             return QueryableHelpers.GroupBy(query, type, groupBuilder.Parts, groupBuilder.Type, groupBuilder.EqualityComparerType);
         }
+
+        public static IQueryable Select(this IQueryable query, Action<SelectBuilder> callback)
+        {
+            var sb = new SelectBuilder();
+            callback(sb);
+            if (sb.Empty)
+                throw new Exception("No select specified, please specify at least one select path");
+
+            return query;
+        }
     }
 }

PoweredSoft.DynamicLinq/Fluent/Select/SelectBuilder.cs

@@ -1,11 +1,112 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Text;
 
 namespace PoweredSoft.DynamicLinq.Fluent
 {
+    public class SelectPart
+    {
+        public string Path { get; set; }
+        public string PropertyName { get; set; }
+        public SelectTypes SelectType { get; set; }
+    }
+
     public class SelectBuilder
     {
+        public List<SelectPart> Parts = new List<SelectPart>();
 
+        public bool Empty => Parts?.Count == 0;
+
+        protected void throwIfUsedOrEmpty(string propertyName)
+        {
+            if (string.IsNullOrEmpty(propertyName))
+                throw new ArgumentNullException($"{propertyName} cannot end up be empty.");
+
+            if (Parts.Any(t => t.PropertyName == propertyName))
+                throw new Exception($"{propertyName} is already used");
+        }
+
+        public SelectBuilder Key(string propertyName, string path = null)
+        {
+            if (propertyName == null)
+                propertyName = path.Split('.').LastOrDefault();
+
+            throwIfUsedOrEmpty(propertyName);
+
+            Parts.Add(new SelectPart
+            {
+                Path = path == null ? "Key" : $"Key.{path}",
+                PropertyName = propertyName,
+                SelectType = SelectTypes.Key
+            });
+
+            return this;
+        }
+
+        public SelectBuilder Count(string propertyName)
+        {
+            throwIfUsedOrEmpty(propertyName);
+            Parts.Add(new SelectPart
+            {
+                PropertyName = propertyName,
+                SelectType = SelectTypes.Count
+            });
+            return this;
+        }
+
+        public SelectBuilder LongCount(string propertyName)
+        {
+            throwIfUsedOrEmpty(propertyName);
+            Parts.Add(new SelectPart
+            {
+                PropertyName = propertyName,
+                SelectType = SelectTypes.LongCount
+            });
+            return this;
+        }
+
+        public SelectBuilder Sum(string path, string propertyName = null)
+        {
+            if (propertyName == null)
+                propertyName = path.Split('.').LastOrDefault();
+
+            throwIfUsedOrEmpty(propertyName);
+
+            Parts.Add(new SelectPart
+            {
+                Path = path,
+                PropertyName = propertyName,
+                SelectType = SelectTypes.Sum
+            });
+            return this;
+        }
+
+        public SelectBuilder Average(string path, string propertyName = null)
+        {
+            if (propertyName == null)
+                propertyName = path.Split('.').LastOrDefault();
+
+            throwIfUsedOrEmpty(propertyName);
+
+            Parts.Add(new SelectPart
+            {
+                Path = path,
+                PropertyName = propertyName,
+                SelectType = SelectTypes.Average
+            });
+            return this;
+        }
+
+        public SelectBuilder ToList(string propertyName)
+        {
+            throwIfUsedOrEmpty(propertyName);
+            Parts.Add(new SelectPart
+            {
+                PropertyName = propertyName,
+                SelectType = SelectTypes.ToList
+            });
+            return this;
+        }
     }
 }