ジェネリック数学（概要）とジェネリック属性（関連する場合）

C# 6でのジェネリック数値演算

Ops-providerパターン

using System;
using System.Collections.Generic;

public interface INumericOps<T>
{
  T Zero { get; }
  T Add(T a, T b);
}

public sealed class IntOps : INumericOps<int>
{
  public int Zero { get { return 0; } }
  public int Add(int a, int b) { return a + b; }
}

public sealed class DoubleOps : INumericOps<double>
{
  public double Zero { get { return 0.0; } }
  public double Add(double a, double b) { return a + b; }
}

public class Program
{
  static T Sum<T>(IEnumerable<T> items, INumericOps<T> ops)
  {
    T acc = ops.Zero;
    foreach (T x in items) acc = ops.Add(acc, x);
    return acc;
  }

  public static void Main(string[] args)
  {
    int s1 = Sum(new int[] {1,2,3}, new IntOps());
    double s2 = Sum(new double[] {1.5, 2.0}, new DoubleOps());
    Console.WriteLine("Sum int = " + s1);
    Console.WriteLine("Sum double = " + s2);
  }
}

ヘルパーを使った平均値

using System;
using System.Collections.Generic;

public interface INumericOps<T>
{
  T Zero { get; }
  T Add(T a, T b);
}

public sealed class DoubleOps : INumericOps<double>
{
  public double Zero { get { return 0.0; } }
  public double Add(double a, double b) { return a + b; }
}

public class Program
{
  static double Average<T>(IEnumerable<T> items, INumericOps<T> ops, Func<T,double> toDouble)
  {
    T acc = ops.Zero;
    int count = 0;
    foreach (T x in items) { acc = ops.Add(acc, x); count++; }
    if (count == 0) throw new ArgumentException("Empty sequence", "items");
    // convert with a delegate for division
    return toDouble(acc) / count;
  }

  public static void Main(string[] args)
  {
    double avg = Average<double>(new double[] {2.0, 4.0, 6.0}, new DoubleOps(), delegate(double d) { return d; });
    Console.WriteLine("Avg = " + avg);
  }
}

デリゲートによる畳み込み

using System;

public class Program
{
  static T Fold<T>(T[] xs, T seed, Func<T,T,T> combine)
  {
    T acc = seed;
    for (int i = 0; i < xs.Length; i++) acc = combine(acc, xs[i]);
    return acc;
  }

  public static void Main(string[] args)
  {
    int sum = Fold<int>(new int[] {1,2,3}, 0, delegate(int a, int b) { return a + b; });
    double sumD = Fold<double>(new double[] {1.5, 2.5}, 0.0, delegate(double a, double b) { return a + b; });
    Console.WriteLine("Sum ints = " + sum);
    Console.WriteLine("Sum doubles = " + sumD);
  }
}

属性の回避策

using System;
using System.Reflection;

// C# 6 cannot declare generic attributes. Workaround: pass a Type.
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public sealed class ValidatorForAttribute : Attribute
{
  public Type TargetType { get; private set; }
  public ValidatorForAttribute(Type targetType) { TargetType = targetType; }
}

public sealed class Customer { public string Name; }

[ValidatorFor(typeof(Customer))]
public sealed class CustomerValidator
{
  public bool IsValid(Customer c) { return c != null && !string.IsNullOrEmpty(c.Name); }
}

public class Program
{
  public static void Main(string[] args)
  {
    Type t = typeof(CustomerValidator);
    object[] attrs = t.GetCustomAttributes(typeof(ValidatorForAttribute), false);
    if (attrs.Length > 0)
    {
      ValidatorForAttribute a = (ValidatorForAttribute)attrs[0];
      Console.WriteLine("Validator targets: " + a.TargetType.Name);
    }
  }
}

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ジェネリック数値演算の回避策

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