[Serializable]
public struct ComplexNumber : ICloneable, IEquatable<ComplexNumber>
{
    // Methods
    public ComplexNumber(double real, double imaginary);
    public bool Equals(ComplexNumber other);    
    public ComplexNumber Multiply(ComplexNumber complex);
    public ComplexNumber Multiply(double number);
    public ComplexNumber Divide(ComplexNumber number);
    public ComplexNumber Divide(double number);
    public ComplexNumber Add(ComplexNumber number);
    public ComplexNumber Subtract(ComplexNumber complex);
    public IMathematicalMatrix ToMatrix();
    public object Clone();
    public override string ToString();
    // ...
	
    // Properties
    public ComplexNumber Conjugate { get; }
    public double Modulus { get; }
    public double Real { get; set; }
    public double Imaginary { get; set; }    
    public ComplexNumber AdditiveInverse { get; }
    public ComplexNumber Reciprocal { get; }
    public static ComplexNumber AdditiveIdentity { get; }
    public static ComplexNumber MultiplicativeIdentity { get; }
    // ...
	
    // Operators
    public static ComplexNumber operator +(ComplexNumber left, ComplexNumber right);
    public static ComplexNumber operator -(ComplexNumber left, ComplexNumber right);
    public static ComplexNumber operator *(ComplexNumber left, ComplexNumber right);
    public static ComplexNumber operator /(ComplexNumber left, ComplexNumber right);
    public static ComplexNumber operator /(ComplexNumber c1, double d1);
    public static ComplexNumber operator *(ComplexNumber c1, double number);
    public static ComplexNumber operator *(double number, ComplexNumber c1);
    public static bool operator ==(ComplexNumber left, ComplexNumber right);
    public static bool operator !=(ComplexNumber left, ComplexNumber right);
    public static explicit operator string(ComplexNumber complexNumber);
    public static implicit operator ComplexNumber(double real);	
}

A complex number is a number in the form of a + bi where a and b are real numbers, and i called the "imaginary" number. The ComplexNumber class provides a method of representing complex numbers, and performing common operations on them like multiplication, subtraction, and more.

See Wikipedia for more details.