C# 7.1’s default value expressions enhancement: default literal expressions
For using C# 7.1, set Language Version to “C# latest minor version(latest)” or “C# 7.1” in C# project (Advanced Build Settings):
default literal expressions and type inference in C# 7.1
default literal can be used when type of the expression can be inferred. It can be used in:
variable initialisation & assignment
public bool HasValue(string value, out string errorMessage)
{
errorMessage = default;
// Some code.
}
optional parameter – default value declaring
public static TValue GetValueOrDefault<TKey, TValue>(this Dictionary<TKey, TValue> dict, TKey key,
TValue defaultVaule = default)
{
// Some code.
}
method call argument
public IEnumerable<Book> GetAllInStockBooks() => GetBooksAvailableForDisplay(default);
public IEnumerable<Book> GetBooksAvailableForDisplay(Func<Book, bool> validate)
{
// Some code.
}
return statement
public static T GetValueOrDefault<T>(this IDataRecord row, string fieldName)
{
// Some Code.
if (row.IsDBNull(ordinal))
{
return default;
}
// Some Code.
}
Full sample code to explain the use of default value expressions in older versions of C# (before C# 7.1)
Note the use of default value expressions:
- variable initialisation & assignment (errorMessage = default(string) )
- optional parameter – default value declaring (TValue GetValueOrDefault<TKey, TValue>(this Dictionary<TKey, TValue> dict, TKey key, TValue defaultVaule = default(TValue)))
- method call argument (GetBooksAvailableForDisplay(default(Func<Book, bool>)))
- return statement (return default(T))
in the below sample code:
/// <summary>
/// Dictionary Extentions.
/// </summary>
public static class DictionaryExtentions
{
/// <summary>
/// Get Value Or Default.
/// </summary>
/// <typeparam name="TKey">Type Param.</typeparam>
/// <typeparam name="TValue">Type Param.</typeparam>
/// <param name="dict">Dictionary.</param>
/// <param name="key">Key.</param>
/// <param name="defaultVaule">Default value to use.</param>
/// <returns>Value.</returns>
public static TValue GetValueOrDefault<TKey, TValue>(this Dictionary<TKey, TValue> dict, TKey key, TValue defaultVaule = default(TValue))
{
// value will be the result or the default for TValue
TValue result;
if (dict.TryGetValue(key, out result) == false)
{
result = defaultVaule;
}
return result;
}
}
/// <summary>
/// DataReader Extentions.
/// </summary>
public static class DataReaderExtentions
{
/// <summary>
/// Get Value Or Default.
/// </summary>
/// <typeparam name="T">Type Param.</typeparam>
/// <param name="row">DataReader row.</param>
/// <param name="fieldName">Field Name.</param>
/// <returns></returns>
public static T GetValueOrDefault<T>(this IDataRecord row, string fieldName)
{
int ordinal = row.GetOrdinal(fieldName);
if (row.IsDBNull(ordinal))
{
return default(T);
}
return (T)row.GetValue(ordinal);
}
}
/// <summary>
/// CSharp Samples.
/// </summary>
public class CSharp
{
/// <summary>
/// Input string has value or not.
/// </summary>
/// <param name="value">Input string.</param>
/// <param name="errorMessage">Error Message (out parameter).</param>
/// <returns>true or false.</returns>
public bool HasValue(string value, out string errorMessage)
{
errorMessage = default(string);
if (value == null)
{
errorMessage = $"{nameof(value)} is null.";
}
else if (value.Equals(string.Empty))
{
errorMessage = $"{nameof(value)} is empty.";
}
else if (value.Trim().Equals(string.Empty))
{
errorMessage = $"{nameof(value)} contains only whitespaces.";
}
return string.IsNullOrEmpty(errorMessage);
}
/// <summary>
/// Get all in stock books.
/// </summary>
/// <returns></returns>
public IEnumerable<Book> GetAllInStockBooks() => GetBooksAvailableForDisplay(default(Func<Book, bool>));
/// <summary>
/// Books available for display.
/// </summary>
/// <param name="validate">Validate function.</param>
/// <returns>Valid books for display.</returns>
public IEnumerable<Book> GetBooksAvailableForDisplay(Func<Book, bool> validate)
{
IEnumerable<Book> booksAvailableInStock = GetBooks().Where(x => x.InStock);
if (validate == null)
{
return booksAvailableInStock;
}
return booksAvailableInStock.Where(x => validate(x));
}
/// <summary>
/// Is valid book for display.
/// </summary>
/// <param name="book">Book.</param>
/// <returns>Valid: true or false.</returns>
public bool IsValidBookForDisplay(Book book)
{
if (book == null
|| string.IsNullOrWhiteSpace(book.Publisher)
|| string.IsNullOrWhiteSpace(book.BookCategory)
|| string.IsNullOrWhiteSpace(book.Title)
|| book.Id <= 0
|| book.Price <= 0)
{
return false;
}
return true;
}
/// <summary>
/// Get Books.
/// </summary>
private IEnumerable<Book> GetBooks() =>
new List<Book>
{
new Book { Publisher = "", BookCategory = "Web Development", Id = 1, Price = 450, Title = "Book1", InStock = false },
new Book { Publisher = "Apress", BookCategory = "Programming", Id = 2, Price = 250, Title = "Book2", InStock = true }, // valid
new Book { Publisher = "wrox", BookCategory = "", Id = 3, Price = 499, Title = "Book3", InStock = true },
new Book { Publisher = "Apress", BookCategory = "Mobile", Id = 0, Price = 350, Title = "Book5", InStock = true },
new Book { Publisher = "wrox", BookCategory = "ASP.NET", Id = 6, Price = 485, Title = "Book6", InStock = false }, // valid
new Book { Publisher = "Apress", BookCategory = "C#", Id = 7, Price = 0, Title = "Book7", InStock = false },
new Book { Publisher = "Apress", BookCategory = "C#", Id = 9, Price = 500, Title = "", InStock = true },
};
}
/// <summary>
/// Book Model.
/// </summary>
public class Book
{
public string Publisher { get; set; }
public string BookCategory { get; set; }
public int Id { get; set; }
public string Title { get; set; }
public decimal Price { get; set; }
public bool InStock { get; set; }
}
Full sample code to explain the use of C# 7.1’s default literal expressions
Note the use of default literal expressions:
- variable initialisation & assignment (errorMessage = default )
- optional parameter – default value declaring (TValue GetValueOrDefault<TKey, TValue>(this Dictionary<TKey, TValue> dict, TKey key, TValue defaultVaule = default))
- method call argument (GetBooksAvailableForDisplay(default))
- return statement (return default)
in the below sample code:
/// <summary>
/// Dictionary Extentions.
/// </summary>
public static class DictionaryExtentions
{
/// <summary>
/// Get Value Or Default.
/// </summary>
/// <typeparam name="TKey">Type Param.</typeparam>
/// <typeparam name="TValue">Type Param.</typeparam>
/// <param name="dict">Dictionary.</param>
/// <param name="key">Key.</param>
/// <param name="defaultVaule">Default value to use.</param>
/// <returns>Value.</returns>
public static TValue GetValueOrDefault<TKey, TValue>(this Dictionary<TKey, TValue> dict, TKey key,
TValue defaultVaule = default)
{
// value will be the result or the default for TValue
if (dict.TryGetValue(key, out TValue result) == false)
{
result = defaultVaule;
}
return result;
}
}
/// <summary>
/// DataReader Extentions.
/// </summary>
public static class DataReaderExtentions
{
/// <summary>
/// Get Value Or Default.
/// </summary>
/// <typeparam name="T">Type Param.</typeparam>
/// <param name="row">DataReader row.</param>
/// <param name="fieldName">Field Name.</param>
/// <returns></returns>
public static T GetValueOrDefault<T>(this IDataRecord row, string fieldName)
{
int ordinal = row.GetOrdinal(fieldName);
if (row.IsDBNull(ordinal))
{
return default;
}
return (T)row.GetValue(ordinal);
}
}
/// <summary>
/// CSharp7_1 Samples.
/// </summary>
public class CSharp7_1
{
/// <summary>
/// Input string has value or not.
/// </summary>
/// <param name="value">Input string.</param>
/// <param name="errorMessage">Error Message (out parameter).</param>
/// <returns>true or false.</returns>
public bool HasValue(string value, out string errorMessage)
{
errorMessage = default;
if (value == null)
{
errorMessage = $"{nameof(value)} is null.";
}
else if (value.Equals(string.Empty))
{
errorMessage = $"{nameof(value)} is empty.";
}
else if (value.Trim().Equals(string.Empty))
{
errorMessage = $"{nameof(value)} contains only whitespaces.";
}
return string.IsNullOrEmpty(errorMessage);
}
/// <summary>
/// Get all in stock books.
/// </summary>
/// <returns></returns>
public IEnumerable<Book> GetAllInStockBooks() => GetBooksAvailableForDisplay(default);
/// <summary>
/// Books available for display.
/// </summary>
/// <param name="validate">Validate function.</param>
/// <returns>Valid books for display.</returns>
public IEnumerable<Book> GetBooksAvailableForDisplay(Func<Book, bool> validate)
{
IEnumerable<Book> booksAvailableInStock = GetBooks().Where(x => x.InStock);
if (validate == null)
{
return booksAvailableInStock;
}
return booksAvailableInStock.Where(x => validate(x));
}
/// <summary>
/// Is valid book for display.
/// </summary>
/// <param name="book">Book.</param>
/// <returns>Valid: true or false.</returns>
public bool IsValidBookForDisplay(Book book)
{
if (book == null
|| string.IsNullOrWhiteSpace(book.Publisher)
|| string.IsNullOrWhiteSpace(book.BookCategory)
|| string.IsNullOrWhiteSpace(book.Title)
|| book.Id <= 0
|| book.Price <= 0)
{
return false;
}
return true;
}
/// <summary>
/// Get Books.
/// </summary>
private IEnumerable<Book> GetBooks() =>
new List<Book>
{
new Book { Publisher = "", BookCategory = "Web Development", Id = 1, Price = 450, Title = "Book1", InStock = false },
new Book { Publisher = "Apress", BookCategory = "Programming", Id = 2, Price = 250, Title = "Book2", InStock = true }, // valid
new Book { Publisher = "wrox", BookCategory = "", Id = 3, Price = 499, Title = "Book3", InStock = true },
new Book { Publisher = "Apress", BookCategory = "Mobile", Id = 0, Price = 350, Title = "Book5", InStock = true },
new Book { Publisher = "wrox", BookCategory = "ASP.NET", Id = 6, Price = 485, Title = "Book6", InStock = false }, // valid
new Book { Publisher = "Apress", BookCategory = "C#", Id = 7, Price = 0, Title = "Book7", InStock = false },
new Book { Publisher = "Apress", BookCategory = "C#", Id = 9, Price = 500, Title = "", InStock = true },
};
}
/// <summary>
/// Book Model.
/// </summary>
public class Book
{
public string Publisher { get; set; }
public string BookCategory { get; set; }
public int Id { get; set; }
public string Title { get; set; }
public decimal Price { get; set; }
public bool InStock { get; set; }
}
Happy Coding !!!
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