SoftCircuits.FixedWidthParser 3.2.0

Fixed-Width Parser

Install-Package SoftCircuits.FixedWidthParser

Overview

SoftCircuits.FixedWidthParser is a lightweight .NET class for reading and writing fixed-width data files.

Fixed-width files are text files that contain one data record on each line. Fields for each record are not delimited. Instead, each field has a fixed width, or length, and fields are found by their location within the line.

The library includes basic classes for reading and writing fixed-width data files. And it also includes generic classes that automatically map class properties to fixed-width fields. The library also provides many options to control the library's behavior.

Breaking Changes in Version 3.0.0

The method signatures for FixedWidthReader.Read(), FixedWidthReader.ReadAsync(), FixedWidthReader<T>.Read(), and FixedWidthReader<T>.ReadAsync() have changed. We were very reluctant to break existing code, but we believe these changes significantly simplify calling these methods. And the code changes should be very minimal.

The FixedWidthReader.Read() and FixedWidthReader.ReadAsync() now store the values in the Values property:

// FixedWidthReader.Read()
using (FixedWidthReader reader = new(PersonFields, filename))
{
    while (reader.Read())
    {
        string[] values = reader.Values;
    }
}

Similarly, the FixedWidthReader<T>.Read() and FixedWidthReader<T>.ReadAsync() now store the object read in the Item property:

// FixedWidthReader<T>.Read()
using (FixedWidthReader<Person> reader = new(filename))
{
    while (reader.Read())
    {
        Debug.Assert(reader.Item != null);
        Person person = reader.Item;
    }
}

Note: The Item property is guaranteed not to be null when FixedWidthReader<T>.ReadAsync() returns true. However, .NET does not currently support the MemberNotNullWhenAttribute attribute for async methods. So the compiler may generate warnings when nullable reference types are enabled. In this case, you can safely use the null-forgiving operator (!) (or Debug.Assert() as shown above) when this method returns true.

FixedWidthWriter and FixedWidthReader Classes

These classes provide the simplest way to read and write fixed-width data files. The example below writes several rows of data to a fixed-width file and then reads it back.

// Define fixed-width fields
FixedWidthField[] PersonFields = new FixedWidthField[]
{
    new FixedWidthField(5),
    new FixedWidthField(10),
    new FixedWidthField(10),
};

// Write data to disk.
// FixedWidthWriter.Write() is overloaded to also accept string[] and IEnumerable<string>.
using (FixedWidthWriter writer = new(PersonFields, filename))
{
    writer.Write("1", "Bill", "Smith");
    writer.Write("2", "Karen", "Williams");
    writer.Write("3", "Tom", "Phillips");
    writer.Write("4", "Jack", "Carpenter");
    writer.Write("5", "Julie", "Samson");
}

// Read the data from disk
using (FixedWidthReader reader = new(PersonFields, filename))
{
    while (reader.Read())
    {
        // reader.Values contains the values read
    }
}

The code above writes and then reads the following file:

1    Bill      Smith     
2    Karen     Williams  
3    Tom       Phillips  
4    Jack      Carpenter 
5    Julie     Samson    

FixedWidthWriter<T> and FixedWidthReader<T> Classes

These classes are used to write and read objects to and from fixed-width data files. Object properties are automatically mapped to fixed-width fields.

All properties and fields in the class with a FixedWidthField attribute will be written and/or read to the fixed-width file. Note that the members don't have to be strings. All the basic data types are supported, including DateTime and Guid.

// Declare our class with FixedWidthField attributes.
// Members without the FixedWidthField attribute will not be written or read.
private class Product
{
    [FixedWidthField(36)]
    public Guid Id { get; set; }
    [FixedWidthField(12)]
    public string Description { get; set; }
    [FixedWidthField(12)]
    public string Category { get; set; }
    [FixedWidthField(10)]
    public double Rating { get; set; }
}

// Create some data
private readonly List<Product> Products = new List<Product>
{
    new Product { Id = Guid.NewGuid(), Description = "Coffee Table", Category = "Furniture", Rating = 4.5 },
    new Product { Id = Guid.NewGuid(), Description = "Spoons", Category = "Utensils", Rating = 4.2 },
    new Product { Id = Guid.NewGuid(), Description = "Carpet", Category = "Flooring", Rating = 4.5 },
    new Product { Id = Guid.NewGuid(), Description = "Knives", Category = "Utensils", Rating = 4.7 },
    new Product { Id = Guid.NewGuid(), Description = "Recliner", Category = "Furniture", Rating = 4.5 },
    new Product { Id = Guid.NewGuid(), Description = "Floor Tiles", Category = "Flooring", Rating = 4.5 },
};

// Write the data to a file
using (FixedWidthWriter<Product> writer = new(filename))
{
    foreach (var product in Products)
        writer.Write(product);
}

// Read the data back from the file
List<Product> results = new();
using (FixedWidthReader<Product> reader = new(filename))
{
    while (reader.Read())
        results.Add(reader.Item);
}

Here's the file created by the code above:

556348bf-058b-48fe-9a13-8ccc376f2e61Coffee TableFurniture   4.5       
3e00e345-1844-4842-b707-168767fb0d5fSpoons      Utensils    4.2       
aae72fec-80f0-48de-8cc5-cd08268624c9Carpet      Flooring    4.5       
ce2026bf-7401-47b2-a7ab-2202292a4425Knives      Utensils    4.7       
17f78e60-4909-4065-9574-d3f60ea55332Recliner    Furniture   4.5       
2d1d4c05-e05b-43dd-bfd5-c0998c4b8312Floor Tiles Flooring    4.5       

Writing Custom Converters

If you have a class member of a type for which there is no built-in support, or if you want to customize the way a member is formatted, you can supply your own data conversion class.

One example where you might need to do this is for DateTime fields. While the built-in DateTime converter works in many cases, date formats can vary wildly. So you might need a custom data converter to control exactly how these values are formatted.

Data conversion classes must implement the IDataConverter interface, but the easiest way to write a custom data converter in a type-safe manner is to derive your class directly from DataConverter<T>, where T is the type of the member you are converting. This class has two abstract members that you must implement in your derived class: ConvertToString() and TryConvertFromString().

The following code reads and writes Person records, which contain a DateTime property. The BirthDateConverter class is used to provide data conversion support for the DateTime property. This is done by setting the ConverterType property of the FixedWidthField attribute.

// Define the Person class
private class Person
{
    [FixedWidthField(8)]
    public int Id { get; set; }
    [FixedWidthField(12)]
    public string FirstName { get; set; }
    [FixedWidthField(12)]
    public string LastName { get; set; }
    [FixedWidthField(12, ConverterType = typeof(BirthDateConverter))]
    public DateTime BirthDate { get; set; }
}

// Define our date converter class
private class BirthDateConverter : DataConverter<DateTime>
{
    private const string Format = "yyyyMMdd";

    public override string ConvertToString(DateTime value) => value.ToString(Format);

    public override bool TryConvertFromString(string? s, out DateTime value)
    {
        return DateTime.TryParseExact(s, Format, null, System.Globalization.DateTimeStyles.None, out value);
    }
}

// Define some Person data
private readonly List<Person> People = new()
{
    new Person { Id = 1, FirstName = "Bill", LastName = "Smith", BirthDate = new DateTime(1982, 2, 7) },
    new Person { Id = 1, FirstName = "Gary", LastName = "Parker", BirthDate = new DateTime(1989, 8, 2) },
    new Person { Id = 1, FirstName = "Karen", LastName = "Wilson", BirthDate = new DateTime(1978, 6, 24) },
    new Person { Id = 1, FirstName = "Jeff", LastName = "Johnson", BirthDate = new DateTime(1972, 4, 18) },
    new Person { Id = 1, FirstName = "John", LastName = "Carter", BirthDate = new DateTime(1982, 12, 21) },
};

// Write the data to a file
using (FixedWidthWriter<Person> writer = new(filename))
{
    foreach (var person in People)
        writer.Write(person);
}

// Read the data back from the file
List<Person> results = new();
using (FixedWidthReader<Person> reader = new(filename))
{
    while (reader.Read())
        results.Add(reader.Item);
}

Here's the file created by the code above:

1       Bill        Smith       19820207    
1       Gary        Parker      19890802    
1       Karen       Wilson      19780624    
1       Jeff        Johnson     19720418    
1       John        Carter      19821221    

The library defines the following custom data converters that you can use as the converter type.

• CompactDateTimeConverter (DateTime)
• DateOnlyDateTimeConverter (DateTime)
• UniversalDateTimeConverter (DateTime)

Manual Field Mapping

When using the FixedWidthReader<T> and FixedWidthWriter<T> classes, it's possible that you need to work with a class that you cannot modify. In this case, you won't be able to add FixedWidthFieldAttribute attributes to the properties you want to serialize.

You can work around this by manually mapping fields using the FixedWidthReader<T>.MapField() and FixedWidthWriter<T>.MapField() methods. These methods will instruct the library how to map class members to fixed-width columns.

using (FixedWidthWriter<Person> writer = new(filename))
{
    writer.MapField(m => m.Id, 8);
    writer.MapField(m => m.FirstName, 12);
    writer.MapField(m => m.LastName, 12);
    writer.MapField(m => m.BirthDate, 12).SetConverterType(typeof(BirthDateConverter));
}

As you can see from the last line, the MapField() method supports a Fluent interface to set additional mapping properties.

If you call the MapField() method for a property that is already mapped (whether from a previous call to MapField() or from a FixedWidthFieldAttribute attribute), the settings will override the existing map settings. However, if you call MapField() for a property that is not already mapped, that field will be appended as the last fixed-width field. So the order fields are mapped will set the order of the fixed-width fields in the output/input file.

Additional Field Options

Whether you define your fields by declaring an array of FixedWidthFields or using the FixedWidthField attribute, there are a number of field options you can specify.

int Length

Gets or sets the number of characters occupied by this column.

FieldAlignment? Alignment

Gets or sets the column alignment for this field. Leave as null to use the default alignment.

char? PadCharacter

Gets or sets the character used to pad this field when writing values shorter than the field width. Leave as null to use the default pad character.

bool? TrimField

Gets or sets whether leading and trailing pad characters are trimmed when reading field values. Leave as null to use the default trim setting.

WARNING: If this property is <c>true</c> and the field value contains leading or trailing characters that match the current pad character, those characters will also be trimmed.

int Skip

Gets or sets the number of characters to skip before the field. Normally, this property is set to zero. You can use this property to skip fixed-width fields that you don't want to read. When writing fixed-width files, the character specified by FixedWidthOptions.DefaultPadCharacter will be written to fill the skipped characters. The default value is 0.

Type ConverterType

Gets or sets the data type that converts this field to and from a string (as demonstrated previously). Must derive from IDataConverter. For best results and type safety, derive the class from DataConverter<T>.

This property is available with the FixedWidthField attribute and not the FixedWidthField class.

Customizing the Library

All of the constructors for the FixedWidthReader, FixedWidthReader<T>, FixedWidthWriter, and FixedWidthWriter<T> class have an optional FixedWidthOptions parameter. To use the default options, leave this parameter as null. Provide your own instance of this class to customize the library settings.

// Set to right align, tilde padding and don't throw exception for invalid data
FixedWidthOptions options = new FixedWidthOptions
{
    DefaultAlignment = FieldAlignment.Right,
    DefaultPadCharacter = '~',
    ThrowDataException = false
};

using (FixedWidthWriter<Product> writer = new FixedWidthWriter<Product>(filename, options))
{
    foreach (var product in Products)
        writer.Write(product);
}

The FixedWidthOptions class has the following properties.

FieldAlignment DefaultAlignment

Gets or sets the default way fields are padded. For example, if a field is right aligned, values shorter than the field width are padded on the left. Can be overridden for individual fields using the FixedWidthField.Alignment property. The default value is FieldAlignment.Left.

char DefaultPadCharacter

Gets or sets the default character used to pad fields when writing values shorter than the field width. Can be overridden for individual fields using the FixedWidthField.PadCharacter property. The default value is ' '.

bool TrimFields

Gets or sets whether leading and trailing pad characters are trimmed when reading field values. Can be overridden for individual fields using the FixedWidthField.TrimField property. The default value is true.

WARNING: If this property is <c>true</c> and the field value contains leading or trailing characters that match the current pad character, those characters will also be trimmed.

bool ThrowDataException

Gets or sets whether a FixedWidthDataException is thrown when reading a field that cannot be converted to the target field type. The default value is true.

bool ThrowOutOfRangeException

Gets or sets whether a FixedWidthOutOfRangeException is thrown when reading a field from a line that is too short. If false, the library reads as much of the field as possible or returns an empty string. The default value is true.

bool ThrowOverflowException

Gets or sets whether a FixedWidthOverflowException is thrown when attempting to write a value that is too large for the field. If false, the value will be silently truncated. The default value is true.