gistfile1.cs

public static Expression<Func<TEntity, TReturn>> GetFieldExpression<TEntity, TReturn>(string fieldName)
{
    var type = typeof(TEntity);
    var member = type.GetFields(BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance)
        .FirstOrDefault(x => x.Name == fieldName);

    if (member == null)
        throw new UnknownPropertyException(type, fieldName);

    var param = Expression.Parameter(member.DeclaringType, "x");
    Expression expression = Expression.Field(param, fieldName);

    if (member.FieldType.IsValueType)
        expression = Expression.Convert(expression, typeof(object));

    return (Expression<Func<TEntity, TReturn>>)Expression.Lambda(typeof(Func<TEntity, TReturn>), expression, param);
}

public static Expression<Func<TEntity, object>> RevealField<TEntity>(this IMappingProvider classMap, string fieldName)
{
    return GetFieldExpression<TEntity, object>(fieldName);
}

public static Expression<Func<TEntity, TReturn>> RevealField<TEntity, TReturn>(this IMappingProvider classMap, string fieldName)
{
    return GetFieldExpression<TEntity, TReturn>(fieldName);
}

public static PropertyPart MapField<TEntity>(this ClasslikeMapBase<TEntity> classMap, string fieldName)
{
    return classMap.Map(GetFieldExpression<TEntity, object>(fieldName)).Access.Field();
}

public static IdentityPart MapIdField<TEntity>(this ClassMap<TEntity> classMap, string fieldName)
{
    return classMap.Id(GetFieldExpression<TEntity, object>(fieldName)).Access.Field();
}

public static ComponentPart<TComponent> MapComponentField<TEntity, TComponent>(this ClassMap<TEntity> classMap, string fieldName, Action<ComponentPart<TComponent>> action)
{
    return classMap.Component<TComponent>(GetFieldExpression<TEntity, TComponent>(fieldName), action).Access.Field();
}