diff -urN webserver-clean/HttpServer/HttpForm.cs webserver-modified/HttpServer/HttpForm.cs
--- webserver-clean/HttpServer/HttpForm.cs	2014-01-10 12:08:32.000000000 +0100
+++ webserver-modified/HttpServer/HttpForm.cs	2014-01-18 23:32:04.000000000 +0100
@@ -10,6 +10,9 @@
 		public static readonly HttpForm EmptyForm = new HttpForm("EmptyForm", true);
 
 		private readonly Dictionary<string, HttpFile> _files = new Dictionary<string, HttpFile>();
+        
+        // A lock to prevent concurrent access to the _files dict
+        private object _lock = new object();
 
 		/// <summary>Initializes a form container with the specified name</summary>
 		public HttpForm() : base("form")
@@ -45,7 +48,8 @@
 			if(_files.ContainsKey(file.Name))
 				throw new ArgumentException("File named '" + file.Name + "' already exists!");
 
-			_files.Add(file.Name, file);
+            lock(_lock)
+			    _files.Add(file.Name, file);
 		}
 
 		/// <summary>
@@ -72,16 +76,19 @@
 		/// <exception cref="ArgumentNullException">If name is null or empty</exception>
 		/// <exception cref="InvalidOperationException">If the instance is HttpForm.EmptyForm which cannot be modified</exception>
 		public HttpFile GetFile(string name)
-		{
-			if (_ignoreChanges)
-				throw new InvalidOperationException("Cannot retrieve files from instance HttpForm.EmptyForm.");
-			if(string.IsNullOrEmpty(name))
-				throw new ArgumentNullException("name");
+        {
+            if (_ignoreChanges)
+                throw new InvalidOperationException("Cannot retrieve files from instance HttpForm.EmptyForm.");
+            if (string.IsNullOrEmpty(name))
+                throw new ArgumentNullException("name");
 
-			if(!_files.ContainsKey(name))
-				return null;
+            lock(_lock)
+            {
+                if (!_files.ContainsKey(name))
+                    return null;
 				
-			return _files[name];
+                return _files[name];
+            }
 		}
 
         /// <summary>
@@ -95,10 +102,13 @@
                 if (_files.Count == 0)
                     return EmptyFileCollection;
 
-                List<HttpFile> files = new List<HttpFile>();
-                foreach (HttpFile file in _files.Values)
-                    files.Add(file);
-                return files.AsReadOnly();
+                lock(_lock)
+                {
+                    List<HttpFile> files = new List<HttpFile>();
+                    foreach(HttpFile file in _files.Values)
+                        files.Add(file);
+                    return files.AsReadOnly();
+                }
             }
         }
 
@@ -106,13 +116,16 @@
 
 		/// <summary>Disposes all held HttpFile's and resets values</summary>
 		public override void Clear()
-		{
-			base.Clear();
+        {
+            base.Clear();
 
-			foreach (KeyValuePair<string, HttpFile> pair in _files)
-				pair.Value.Dispose();
+            lock(_lock)
+            {
+                foreach(KeyValuePair<string, HttpFile> pair in _files)
+                    pair.Value.Dispose();
 
-			_files.Clear();
+                _files.Clear();
+            }
 		}
 	}
 }