首先，我按照本教程创建了我的Money对象：https : //www.codeproject.com/articles/837791/money-pattern
Money totalItems = _invoice.InvoiceDetails
.Sum(y => y.Amount); // Amount is of type Money
我在上收到编译异常y.Amount：
无法将类型'Money'隐式转换为'long？' 无法将lambda表达式转换为预期的委托类型，因为该块中的某些返回类型不能隐式转换为委托返回类型
我究竟做错了什么？
这是我的Money课程：
public class Money
{
public decimal Amount { get; private set; }
public CurrencyCode Currency { get; private set; }
#region Constructors
public Money() { }
public Money(Money amount)
{
this.Amount = amount.Amount;
this.Currency = amount.Currency;
}
public Money(decimal amount, CurrencyCode currencyCode)
{
this.Amount = amount;
this.Currency = currencyCode;
}
public Money(int amount, CurrencyCode currency)
: this(Convert.ToDecimal(amount), currency)
{
}
public Money(double amount, CurrencyCode currency)
: this(Convert.ToDecimal(amount), currency)
{
}
#endregion
#region Comprasion operators
public static bool operator ==(Money var1, Money var2)
{
if ((object)var1 == null || (object)var2 == null)
return false;
if (var1.Currency != var2.Currency) return false;
return var1.Amount == var2.Amount;
}
public static bool operator !=(Money var1, Money var2)
{
return !(var1 == var2);
}
public static bool operator >(Money var1, Money var2)
{
if (var1.Currency != var2.Currency)
throw new InvalidOperationException("Comprasion between different currencies is not allowed.");
return var1.Amount > var2.Amount;
}
public static bool operator <(Money var1, Money var2)
{
if (var1 == var2) return false;
return !(var1 > var2);
}
public static bool operator <=(Money var1, Money var2)
{
if (var1 < var2 || var1 == var2) return true;
return false;
}
public static bool operator >=(Money var1, Money var2)
{
if (var1 > var2 || var1 == var2) return true;
return false;
}
#endregion
#region Ariphmetical operations
public static Money operator +(Money var1, Money var2)
{
if (var1.Currency != var2.Currency)
{
throw new InvalidCastException("Calculation is using different currencies!");
}
return new Money(var1.Amount + var2.Amount, var1.Currency);
}
public static Money operator -(Money var1, Money var2)
{
if (var1.Currency != var2.Currency)
{
throw new InvalidCastException("Calculation is using different currencies!");
}
return new Money(var1.Amount - var2.Amount, var1.Currency);
}
public static Money operator *(Money var1, Money var2)
{
if (var1.Currency != var2.Currency)
{
throw new InvalidCastException("Calculation is using different currencies!");
}
return new Money(var1.Amount * var2.Amount, var1.Currency);
}
public static Money operator /(Money var1, Money var2)
{
if (var1.Currency != var2.Currency)
{
throw new InvalidCastException("Calculation is using different currencies!");
}
return new Money(var1.Amount / var2.Amount, var1.Currency);
}
public static Money operator *(decimal var1, Money var2)
{
return new Money(var1 * var2.Amount, var2.Currency);
}
public static Money operator *(Money var1, decimal var2)
{
return new Money(var1.Amount * var2, var1.Currency);
}
public static Money operator /(decimal var1, Money var2)
{
return new Money(var1 / var2.Amount, var2.Currency);
}
public static Money operator /(Money var1, decimal var2)
{
return new Money(var1.Amount / var2, var1.Currency);
}
public static Money operator *(int var1, Money var2)
{
return new Money(var1 * var2.Amount, var2.Currency);
}
public static Money operator *(Money var1, int var2)
{
return new Money(var1.Amount * var2, var1.Currency);
}
public static Money operator /(int var1, Money var2)
{
return new Money(var1 / var2.Amount, var2.Currency);
}
public static Money operator /(Money var1, int var2)
{
return new Money(var1.Amount / var2, var1.Currency);
}
public static Money operator *(long var1, Money var2)
{
return new Money(var1 * var2.Amount, var2.Currency);
}
public static Money operator *(Money var1, long var2)
{
return new Money(var1.Amount * var2, var1.Currency);
}
public static Money operator /(long var1, Money var2)
{
return new Money(var1 / var2.Amount, var2.Currency);
}
public static Money operator /(Money var1, long var2)
{
return new Money(var1.Amount / var2, var1.Currency);
}
#endregion
public override bool Equals(object obj)
{
if (obj == null) return false;
Money money = obj as Money;
return (this.Amount == money.Amount && this.Currency == money.Currency);
}
public bool Equals(Money money)
{
if ((object)money == null) return false;
return (this.Amount == money.Amount && this.Currency == money.Currency);
}
public override int GetHashCode()
{
return base.GetHashCode();
}
public override string ToString()
{
return this.Amount.ToString();
}
#endregion
dbc.. 5
大概InvoiceDetails是包含public Money Amount属性的类的集合，例如：
public class InvoiceDetail
{
public Money Amount { get; set; }
}
在这种情况下，您可以使用Enumerable.Aggregate()做总和：
var sum = InvoiceDetails.Aggregate(new Money(0, InvoiceDetails.First().Amount.Currency), (s, d) => s + d.Amount);
为了摆脱稍微难看的new Money(0, InvoiceDetails.First().Amount.Currency)表情，您可能想要引入一个Money.Empty不包含任何货币并且可以添加到任何类型的货币的特殊单例。或修改静态运算符以接受的null值Money并执行以下操作：
var sum = InvoiceDetails.Aggregate((Money)null, (s, d) => s + d.Amount);
或者，引入中间体Select()可以使表达更清晰：
var sum = InvoiceDetails.Select(d => d.Amount).Aggregate((s, a) => s + a);
Enumerable.Sum()不起作用的原因是它是为固定的一组可枚举的算术类型定义的。没有Sum()引入了算术运算符重载的任意类型，因为在这种情况下没有通用的接口或类型推断。（请参见是否存在将我的泛型方法限制为数值类型的约束？答案为“否”。）当然，您可以添加自己的版本，Enumerable.Sum()以支持提供自己的算术类型，请参见此答案为一个开始的地方。
大概InvoiceDetails是包含public Money Amount属性的类的集合，例如：
public class InvoiceDetail
{
public Money Amount { get; set; }
}
在这种情况下，您可以使用Enumerable.Aggregate()做总和：
var sum = InvoiceDetails.Aggregate(new Money(0, InvoiceDetails.First().Amount.Currency), (s, d) => s + d.Amount);
为了摆脱稍微难看的new Money(0, InvoiceDetails.First().Amount.Currency)表情，您可能想要引入一个Money.Empty不包含任何货币并且可以添加到任何类型的货币的特殊单例。或修改静态运算符以接受的null值Money并执行以下操作：
var sum = InvoiceDetails.Aggregate((Money)null, (s, d) => s + d.Amount);
或者，引入中间体Select()可以使表达更清晰：
var sum = InvoiceDetails.Select(d => d.Amount).Aggregate((s, a) => s + a);
Enumerable.Sum()不起作用的原因是它是为固定的一组可枚举的算术类型定义的。没有Sum()引入了算术运算符重载的任意类型，因为在这种情况下没有通用的接口或类型推断。（请参见是否存在将我的泛型方法限制为数值类型的约束？答案为“否”。）当然，您可以添加自己的版本，Enumerable.Sum()以支持提供自己的算术类型，请参见此答案为一个开始的地方。
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