Set FileHelper's FileHelperEngine.HeaderText via reflection

FileHelpersTypeExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;

// see http://stackoverflow.com/questions/3975741/column-headers-in-csv-using-filehelpers-library/8258420#8258420

// ReSharper disable CheckNamespace
namespace FileHelpers
// ReSharper restore CheckNamespace
{
    [AttributeUsage(AttributeTargets.Field, AllowMultiple = false, Inherited = true)]
    public class FieldTitleAttribute : Attribute
    {
        public FieldTitleAttribute(string name)
        {
            if (name == null) throw new ArgumentNullException("name");
            Name = name;
        }

        public string Name { get; private set; }
    }

    public static class FileHelpersTypeExtensions
    {
        public static IEnumerable<string> GetFieldTitles(this Type type)
        {
            var fields = from field in type.GetFields(
                BindingFlags.GetField |
                BindingFlags.Public |
                BindingFlags.NonPublic |
                BindingFlags.Instance)
                         where field.IsFileHelpersField()
                         select field;

            return from field in fields
                   let attrs = field.GetCustomAttributes(true)
                   let order = attrs.OfType<FieldOrderAttribute>().Single().GetOrder()
                   let title = attrs.OfType<FieldTitleAttribute>().Single().Name
                   orderby order
                   select title;
        }

        public static string GetCsvHeader(this Type type)
        {
            return String.Join(",", type.GetFieldTitles());
        }

        static bool IsFileHelpersField(this FieldInfo field)
        {
            return field.GetCustomAttributes(true)
                .OfType<FieldOrderAttribute>()
                .Any();
        }

        static int GetOrder(this FieldOrderAttribute attribute)
        {
            // Hack cos FieldOrderAttribute.Order is internal (why?)
            var pi = typeof(FieldOrderAttribute)
                .GetProperty("Order",
                    BindingFlags.GetProperty |
                    BindingFlags.Instance |
                    BindingFlags.NonPublic);

            return (int)pi.GetValue(attribute, null);
        }
    }
}

Usage.cs

[DelimitedRecord(","), IgnoreFirst(1)]
public class Person
{
    // Must specify FieldOrder too
    [FieldOrder(1), FieldTitle("Name")]
    string name;

    [FieldOrder(2), FieldTitle("Age")]
    int age;
}

...

var engine = new FileHelperEngine<Person>
{
    HeaderText = typeof(Person).GetCsvHeader()
};

...

engine.WriteFile(@"C:\people.csv", people);

Thanks for this gist Richard!

I found the "FieldTitle" attribute to be a bit redundant, so I just use the field name instead:

public static class FileHelperUtils {
    public static IEnumerable<string> GetFieldTitles(this Type type) 
    {
        var fields = from field in type.GetFields(
            BindingFlags.GetField |
            BindingFlags.Public |
            BindingFlags.NonPublic |
            BindingFlags.Instance)
                     where field.IsFileHelpersField()
                     select field;

        return from field in fields
               let attrs = field.GetCustomAttributes(true)
               let order = attrs.OfType<FieldOrderAttribute>().Single().GetOrder()
               let title = field.Name
               orderby order
               select title;
    }

    public static string GetCsvHeader(this Type type)
    {
        return String.Join(",", type.GetFieldTitles());
    }

    static bool IsFileHelpersField(this FieldInfo field)
    {
        return field.GetCustomAttributes(true)
            .OfType<FieldOrderAttribute>()
            .Any();
    }

    static int GetOrder(this FieldOrderAttribute attribute)
    {
        // Hack cos FieldOrderAttribute.Order is internal (why?)
        var pi = typeof(FieldOrderAttribute)
            .GetProperty("Order",
                BindingFlags.GetProperty |
                BindingFlags.Instance |
                BindingFlags.NonPublic);

        return (int)pi.GetValue(attribute, null);
    }
}

Np. We had to use FieldTitle because we were using property-backed fields and wanted uppercase column names in the CSV (the version of FileHelpers didn't support properties... not sure how this is possible in 2011)

Additional option: use a delimiter as seperator. Too bad the original delimiter is internal, so I had to create a custom class; Joiner.

Code:

    public static string GetCsvHeader(this Type type) {
        return String.Join(type.GetCustomAttributes(true).OfType<Joiner>().Single().Delimiter, type.GetFieldTitles());
    }

[AttributeUsage(AttributeTargets.Class)]
public sealed class Joiner : Attribute {
    public string Delimiter;

    /// <summary>Indicates that this class represents a delimited record. </summary>
    /// <param name="delimiter">The separator string used to split the fields of the record.</param>
    public Joiner(string delimiter) {
            this.Delimiter = delimiter;
    }
}

@FWest98

1. This is covered by the engine: use engine.GetFileHeader()
   see https://github.com/MarcosMeli/FileHelpers/blob/master/FileHelpers/Engines/EngineBase.cs
2. DelimitedRecord has (now) a public accessible property Separator MarcosMeli/FileHelpers@f46a39a
   => type.GetCustomAttributes(true).OfType<DelimitedRecordAttribute>()
3. in older versions, you should be able to access this via DelimitedFileEngine(type).Options.Delimiter

so, no need for an additional attribute ;)

overall:
engine.GetFileHeader() seems to use field.FriendlyName from FieldBase, but I can't see how or if this can be invoked? Default is fieldName, else a part of the reflective field ... I don't understand that code atm. So perhaps @MarcosMeli or someone else might point it out ;) Perhaps it's much easier to add custom fieldTitles?!

I would really add BindingFlags.Public to

    static int GetOrder(this FieldOrderAttribute attribute)
    {
        // Hack cos FieldOrderAttribute.Order is internal (why?)
        var pi = typeof(FieldOrderAttribute)
            .GetProperty("Order",
                BindingFlags.GetProperty |
                BindingFlags.Instance |
                BindingFlags.NonPublic);

        return (int)pi.GetValue(attribute, null);
    }

What is FieldOrderAttribute stands for?

https://github.com/MarcosMeli/FileHelpers/blob/master/FileHelpers/Attributes/FieldOrderAttribute.cs

Thrown Error in FieldTitleAttribute

Given attribute as

    [FieldOrder(1), FieldTitle("Field1")]
    [FieldTrim(TrimMode.Both)]
    public String Field1;

    [FieldOrder(2), FieldTitle("Field2")]
    public Int32 Field2;

    [FieldOrder(3), FieldTitle("Field3")]
    public Int32 Field3;

    [FieldOrder(4), FieldTitle("Field4")]
    [FieldTrim(TrimMode.Both)]
    [FieldConverter(ConverterKind.Date, "dd/MM/yyyy")]
    public DateTime Field4;

    [FieldOrder(5), FieldTitle("Field5")]
    [FieldTrim(TrimMode.Both)]
    public String Field5;

    [FieldOrder(6), FieldTitle("Field6")]
    [FieldTrim(TrimMode.Both)]
    public String Field6;

Got Error as :

System.NullReferenceException was unhandled
HResult=-2147467261
Message=Object reference not set to an instance of an object.
Source=ConsoleApplicationBRDTest
StackTrace:
at FileHelpers.FileHelpersTypeExtensions.GetOrder(FieldOrderAttribute attribute) in C:\Users\T3328\documents\visual studio 2010\Projects\ConsoleApplicationBRDTest\ConsoleApplicationBRDTest\FileHelpersTypeExtensions.cs:line 67
at FileHelpers.FileHelpersTypeExtensions.b__5(<>f__AnonymousType02 <>h__TransparentIdentifier0) in C:\Users\T3328\documents\visual studio 2010\Projects\ConsoleApplicationBRDTest\ConsoleApplicationBRDTest\FileHelpersTypeExtensions.cs:line 40 at System.Linq.Enumerable.<>c__DisplayClass123.b__11(TSource x)
at System.Linq.Enumerable.<>c__DisplayClass123.<CombineSelectors>b__11(TSource x) at System.Linq.Enumerable.WhereSelectArrayIterator2.MoveNext()
at System.Linq.Buffer1..ctor(IEnumerable1 source)
at System.Linq.OrderedEnumerable1.<GetEnumerator>d__0.MoveNext() at System.Linq.Enumerable.WhereSelectEnumerableIterator2.MoveNext()
at System.String.Join(String separator, IEnumerable`1 values)
at FileHelpers.FileHelpersTypeExtensions.GetCsvHeader(Type type) in C:\Users\T3328\documents\visual studio 2010\Projects\ConsoleApplicationBRDTest\ConsoleApplicationBRDTest\FileHelpersTypeExtensions.cs:line 48
at ConsoleApplicationBRDTest.Program.Main(String[] args) in C:\Users\T3328\documents\visual studio 2010\Projects\ConsoleApplicationBRDTest\ConsoleApplicationBRDTest\Program.cs:line 102
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
InnerException:

when returning line : return (int)pi.GetValue(attribute, null);

Will it be correct if I replace like this :

//return (int)pi.GetValue(attribute,null);

return attribute.Order;

VB.NET version should anyone need this:-

Imports System.Reflection
Imports FileHelpers

Namespace FileHelpers
    ' Thanks to Richard Dingwall @ https://gist.github.com/rdingwall/1391429
    Public Module FileHelpersTypeExtensions
        <Runtime.CompilerServices.Extension> _
        Public Function GetFieldTitles(type As Type) As IEnumerable(Of String)
            Dim fields = From field In type.GetFields(
                BindingFlags.GetField Or 
                BindingFlags.Public Or 
                BindingFlags.NonPublic Or 
                BindingFlags.Instance)
                    Where field.IsFileHelpersField()
                    Select field

            Return From field In fields
                Let attrs = field.GetCustomAttributes(True)
                Let order = attrs.OfType(Of FieldOrderAttribute)().Single().GetOrder()
                Let title = attrs.OfType(Of FieldTitleAttribute)().Single().Name
                Order By order
                Select title
        End Function

        <Runtime.CompilerServices.Extension> _
        Public Function GetCsvHeader(type As Type) As String
            Return String.Join(",", type.GetFieldTitles())
        End Function

        <Runtime.CompilerServices.Extension> _
        Private Function IsFileHelpersField(field As FieldInfo) As Boolean
            Return field.GetCustomAttributes(True).OfType(Of FieldOrderAttribute)().Any()
        End Function

        <Runtime.CompilerServices.Extension> _
        Private Function GetOrder(attribute As FieldOrderAttribute) As Integer
            ' Hack cos FieldOrderAttribute.Order is internal (why?)
            Dim pi = GetType(FieldOrderAttribute).GetProperty(
                "Order", 
                BindingFlags.GetProperty Or 
                BindingFlags.Instance Or 
                BindingFlags.Public Or 
                BindingFlags.NonPublic)

            Return CInt(pi.GetValue(attribute, Nothing))
        End Function
    End Module
End NameSpace

<DelimitedRecord(","), IgnoreFirst(1)>
Public Class Person
     <FieldOrder(1), FieldTitle("Name")>
     Public Name As String
     <FieldOrder(2), FieldTitle("Age")>
     Public Age As Integer
End Class

Dim importEngine = New FileHelperEngine(Of Person)() With {
     .HeaderText = GetType(Person).GetCsvHeader()
}

GetOrder does not work with latest version of FileHelpers.
In older version Order property was internal and it was access using reflection (GetOrder method), with new version Order property is now public (our GetOrder method does not cater to public properties) so it can be accessed directly.

Code Changes:

1. Get rid of GetOrder method
2. Update
   Let order = attrs.OfType(Of FieldOrderAttribute)().Single().GetOrder()
   to
   Let order = attrs.OfType(Of FieldOrderAttribute)().Single().Order

Just stumbled upon this and did some changes to make it work with my use case.
This should work with both fields and properties.
Don't know if this is fixed in the original repo by now.

Input:

    [DelimitedRecord(",") , IgnoreFirst(1)]
    public class SystemConfig
    {
        [FieldOrder(1), FieldTitle("Id")]
        public int? Id { get; set; }

        [FieldOrder(2), FieldTitle("Key")]
        public string Key { get; set; }

        [FieldOrder(3), FieldTitle("ApplicationName")]
        public string ApplicationName { get; set; }

        [FieldOrder(4), FieldTitle("ApplicationModule")]
        public string ApplicationModule { get; set; }

        [FieldOrder(5), FieldTitle("ServerName")]
        public string ServerName { get; set; }

        [FieldOrder(6), FieldTitle("ValueString")]
        public string ValueString { get; set; }
   }

Edited code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;

namespace FileHelpers
{
    [AttributeUsage(AttributeTargets.Property | AttributeTargets.Field, AllowMultiple = false, Inherited = true)]
    public class FieldTitleAttribute : Attribute
    {
        public FieldTitleAttribute(string name)
        {
            Name = name ?? throw new ArgumentNullException(nameof(name));
        }

        public string Name { get; private set; }
    }

    public static class FileHelpersTypeExtensions
    {
        public static IEnumerable<string> GetFieldTitles(this Type type)
        {

            var props = from prop in type.GetProperties(
                    BindingFlags.GetProperty |
                    BindingFlags.Public |
                    BindingFlags.NonPublic |
                    BindingFlags.Instance)
                where prop.IsFileHelpersProperty()
                select prop;

            return  from prop in props
                let attrs = prop.GetCustomAttributes(true)
                let order = attrs.OfType<FieldOrderAttribute>().Single().Order
                let title = attrs.OfType<FieldTitleAttribute>().Single().Name
                orderby order
                select title;
        }

        public static string GetCsvHeader(this Type type)
        {
            return String.Join(",", type.GetFieldTitles());
        }

        static bool IsFileHelpersProperty(this PropertyInfo prop)
        {
            var tmp =  prop.GetCustomAttributes(true)
                .OfType<FieldOrderAttribute>()
                .Any();

            return tmp;
        }
    }
}