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    <p>I'm at it again!<br> Here's the solution to all your <code>List&lt;T&gt;</code> and <code>Enumerable&lt;T&gt;</code> conversion problems. ~150 lines of code<br> Just be sure to define at least one explicit or implicit conversion operator for the input/output types involved (if one does not exist), as you should be doing anyway!</p> <pre><code>using System; using System.Collections; using System.Collections.Generic; using System.Linq; using System.Text; using System.Reflection; namespace System.Collections.Generic //purposely in same namespace as List&lt;T&gt;,IEnumerable&lt;T&gt;, so extension methods are available with them { public static class Enumerable { public static List&lt;TOutput&gt; ConvertAll&lt;TInput,TOutput&gt;( this IEnumerable&lt;TInput&gt; input ) { return BuildConvertedList&lt;TInput,TOutput&gt;( input, GetConverterDelegate&lt;TInput,TOutput&gt;() ); } public static IEnumerable&lt;TOutput&gt; ConvertAll&lt;TInput,TOutput&gt;( this IEnumerable&lt;TInput&gt; input, bool lazy ) { if (lazy) return new LazyConverter&lt;TInput,TOutput&gt;( input, GetConverterDelegate&lt;TInput,TOutput&gt;() ); return BuildConvertedList&lt;TInput,TOutput&gt;( input, GetConverterDelegate&lt;TInput,TOutput&gt;() ); } public static List&lt;TOutput&gt; ConvertAll&lt;TInput,TOutput&gt;( this IEnumerable&lt;TInput&gt; input, Converter&lt;TInput, TOutput&gt; converter ) { return BuildConvertedList&lt;TInput,TOutput&gt;( input, converter ); } public static List&lt;TOutput&gt; ConvertAll&lt;TInput, TOutput&gt;( this List&lt;TInput&gt; input ) { Converter&lt;TInput, TOutput&gt; converter = GetConverterDelegate&lt;TInput,TOutput&gt;(); return input.ConvertAll&lt;TOutput&gt;( converter ); } public static IEnumerable&lt;TOutput&gt; ConvertAll&lt;TInput, TOutput&gt;( this List&lt;TInput&gt; input, Converter&lt;TInput, TOutput&gt; converter, bool lazy ) { if (lazy) return new LazyConverter&lt;TInput, TOutput&gt;( input, converter ); return input.ConvertAll&lt;TOutput&gt;( converter ); } public static List&lt;TOutput&gt; ConvertAll&lt;TInput, TOutput&gt;( this List&lt;TInput&gt; input, Converter&lt;TInput, TOutput&gt; converter ) { return input.ConvertAll&lt;TOutput&gt;( converter ); } //Used to manually build converted list when input is IEnumerable, since it doesn't have the ConvertAll method like the List does private static List&lt;TOutput&gt; BuildConvertedList&lt;TInput,TOutput&gt;( IEnumerable&lt;TInput&gt; input, Converter&lt;TInput, TOutput&gt; converter ){ List&lt;TOutput&gt; output = new List&lt;TOutput&gt;(); foreach (TInput input_item in input) output.Add( converter( input_item ) ); return output; } private sealed class LazyConverter&lt;TInput, TOutput&gt;: IEnumerable&lt;TOutput&gt;, IEnumerator&lt;TOutput&gt; { private readonly IEnumerable&lt;TInput&gt; input; private readonly Converter&lt;TInput, TOutput&gt; converter; private readonly IEnumerator&lt;TInput&gt; input_enumerator; public LazyConverter( IEnumerable&lt;TInput&gt; input, Converter&lt;TInput, TOutput&gt; converter ) { this.input = input; this.converter = converter; this.input_enumerator = input.GetEnumerator(); } public IEnumerator&lt;TOutput&gt; GetEnumerator() {return this;} //IEnumerable&lt;TOutput&gt; Member IEnumerator IEnumerable.GetEnumerator() {return this;} //IEnumerable Member public void Dispose() {input_enumerator.Dispose();} //IDisposable Member public TOutput Current {get {return converter.Invoke( input_enumerator.Current );}} //IEnumerator&lt;TOutput&gt; Member object IEnumerator.Current {get {return Current;}} //IEnumerator Member public bool MoveNext() {return input_enumerator.MoveNext();} //IEnumerator Member public void Reset() {input_enumerator.Reset();} //IEnumerator Member } private sealed class TypeConversionPair: IEquatable&lt;TypeConversionPair&gt; { public readonly Type source_type; public readonly Type target_type; private readonly int hashcode; public TypeConversionPair( Type source_type, Type target_type ) { this.source_type = source_type; this.target_type = target_type; //precalc/store hash, since object is immutable; add one to source hash so reversing the source and target still produces unique hash hashcode = (source_type.GetHashCode() + 1) ^ target_type.GetHashCode(); } public static bool operator ==( TypeConversionPair x, TypeConversionPair y ) { if ((object)x != null) return x.Equals( y ); if ((object)y != null) return y.Equals( x ); return true; //x and y are both null, cast to object above ensures reference equality comparison } public static bool operator !=( TypeConversionPair x, TypeConversionPair y ) { if ((object)x != null) return !x.Equals( y ); if ((object)y != null) return !y.Equals( x ); return false; //x and y are both null, cast to object above ensures reference equality comparison } //TypeConversionPairs are equal when their source and target types are equal public bool Equals( TypeConversionPair other ) { if ((object)other == null) return false; //cast to object ensures reference equality comparison return source_type == other.source_type &amp;&amp; target_type == other.target_type; } public override bool Equals( object obj ) { TypeConversionPair other = obj as TypeConversionPair; if ((object)other != null) return Equals( other ); //call IEqualityComparer&lt;TypeConversionPair&gt; implementation if obj type is TypeConversionPair return false; //obj is null or is not of type TypeConversionPair; Equals shall not throw errors! } public override int GetHashCode() {return hashcode;} //assigned in constructor; object is immutable } private static readonly Dictionary&lt;TypeConversionPair,Delegate&gt; conversion_op_cache = new Dictionary&lt;TypeConversionPair,Delegate&gt;(); //Uses reflection to find and create a Converter&lt;TInput, TOutput&gt; delegate for the given types. //Once a delegate is obtained, it is cached, so further requests for the delegate do not use reflection* //(*the typeof operator is used twice to look up the type pairs in the cache) public static Converter&lt;TInput, TOutput&gt; GetConverterDelegate&lt;TInput, TOutput&gt;() { Delegate converter; TypeConversionPair type_pair = new TypeConversionPair( typeof(TInput), typeof(TOutput) ); //Attempt to quickly find a cached conversion delegate. lock (conversion_op_cache) //synchronize with concurrent calls to Add if (conversion_op_cache.TryGetValue( type_pair, out converter )) return (Converter&lt;TInput, TOutput&gt;)converter; //Get potential conversion operators (target-type methods are ordered first) MethodInfo[][] conversion_op_sets = new MethodInfo[2][] { type_pair.target_type.GetMethods( BindingFlags.Static | BindingFlags.Public | BindingFlags.FlattenHierarchy ), type_pair.source_type.GetMethods( BindingFlags.Static | BindingFlags.Public | BindingFlags.FlattenHierarchy ) }; //Find appropriate conversion operator, //favoring operators on target type in case functionally equivalent operators exist, //since the target type's conversion operator may have access to an appropriate constructor //or a common instance cache (i.e. immutable objects may be cached and reused). for (int s = 0; s &lt; conversion_op_sets.Length; s++) { MethodInfo[] conversion_ops = conversion_op_sets[s]; for (int m = 0; m &lt; conversion_ops.Length; m++) { MethodInfo mi = conversion_ops[m]; if ((mi.Name == "op_Explicit" || mi.Name == "op_Implicit") &amp;&amp; mi.ReturnType == type_pair.target_type &amp;&amp; mi.GetParameters()[0].ParameterType.IsAssignableFrom( type_pair.source_type )) //Assuming op_Explicit and op_Implicit always have exactly one parameter. { converter = Delegate.CreateDelegate( typeof(Converter&lt;TInput, TOutput&gt;), mi ); lock (conversion_op_cache) //synchronize with concurrent calls to TryGetValue conversion_op_cache.Add( type_pair, converter ); //Cache the conversion operator reference for future use. return (Converter&lt;TInput, TOutput&gt;)converter; } } } return (TInput x) =&gt; ((TOutput)Convert.ChangeType( x, typeof(TOutput) )); //this works well in the absence of conversion operators for types that implement IConvertible //throw new InvalidCastException( "Could not find conversion operator to convert " + type_pair.source_type.FullName + " to " + type_pair.target_type.FullName + "." ); } } } </code></pre> <p><strong>Sample use:</strong></p> <pre><code>using System; using System.Collections.Generic; namespace ConsoleApplication1 { class Program { static void Main(string[] args) { List&lt;string&gt; list = new List&lt;string&gt;(new string[] { "abcde", "abcd", "abc"/*will break length constraint*/, "ab", "a" }); //Uncomment line below to see non-lazy behavior. All items converted before method returns, and will fail on third item, which breaks the length constraint. //List&lt;ConstrainedString&gt; constrained_list = list.ConvertAll&lt;string,ConstrainedString&gt;(); IEnumerable&lt;ConstrainedString&gt; constrained_list = list.ConvertAll&lt;string,ConstrainedString&gt;( true ); //lazy conversion; conversion is not attempted until that item is read foreach (ConstrainedString constrained_string in constrained_list) //will not fail until the third list item is read/converted System.Console.WriteLine( constrained_string.ToString() ); } public class ConstrainedString { private readonly string value; public ConstrainedString( string value ){this.value = Constrain(value);} public string Constrain( string value ) { if (value.Length &gt; 3) return value; throw new ArgumentException("String length must be &gt; 3!"); } public static explicit operator ConstrainedString( string value ){return new ConstrainedString( value );} public override string ToString() {return value;} } } } </code></pre>
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