修改版。这个类可能比你可以找到的许多其他类更好:-) 这是因为这个版本支持直接属性（p => p.B）（和其他人一样:-)）、嵌套属性（p => p.B.C.D）、字段（两者都是“终端”和“中间”，所以在p => p.B.C.D 中B 和D 都可以是字段）和类型的“内部”转换（所以p => ((BType)p.B).C.D 和p => (p.B as BType).C.D)。唯一不是支持的是“终端”元素的转换（所以没有p => (object)p.B）。
生成器中有两个“代码路径”：用于简单表达式 (p => p.B) 和用于“嵌套”表达式。 .NET 4.0 有代码变体（具有 Expression.Assign 表达式类型）。从我的一些基准测试中，最快的代表是：“简单”Delegate.CreateDelegate 用于属性，Expression.Assign 用于字段，“简单”FieldSetter 用于字段（这个比Expression.Assign 字段慢一点）。所以在.NET 4.0下你应该去掉所有标记为3.5的代码。
部分代码不是我的。初始（简单）版本基于 Fluent NHibernate 代码（但它仅支持直接属性），其他一些部分基于来自 How do I set a field value in an C# Expression tree? 和 Assignment in .NET 3.5 expression trees 的代码。
public static class FluentTools
{
public static Action<T, TValue> GetterToSetter<T, TValue>(Expression<Func<T, TValue>> getter)
{
ParameterExpression parameter;
Expression instance;
MemberExpression propertyOrField;
GetMemberExpression(getter, out parameter, out instance, out propertyOrField);
// Very simple case: p => p.Property or p => p.Field
if (parameter == instance)
{
if (propertyOrField.Member.MemberType == MemberTypes.Property)
{
// This is FASTER than Expression trees! (5x on my benchmarks) but works only on properties
PropertyInfo property = propertyOrField.Member as PropertyInfo;
MethodInfo setter = property.GetSetMethod();
var action = (Action<T, TValue>)Delegate.CreateDelegate(typeof(Action<T, TValue>), setter);
return action;
}
#region .NET 3.5
else // if (propertyOrField.Member.MemberType == MemberTypes.Field)
{
// 1.2x slower than 4.0 method, 5x faster than 3.5 method
FieldInfo field = propertyOrField.Member as FieldInfo;
var action = FieldSetter<T, TValue>(field);
return action;
}
#endregion
}
ParameterExpression value = Expression.Parameter(typeof(TValue), "val");
Expression expr = null;
#region .NET 3.5
if (propertyOrField.Member.MemberType == MemberTypes.Property)
{
PropertyInfo property = propertyOrField.Member as PropertyInfo;
MethodInfo setter = property.GetSetMethod();
expr = Expression.Call(instance, setter, value);
}
else // if (propertyOrField.Member.MemberType == MemberTypes.Field)
{
expr = FieldSetter(propertyOrField, value);
}
#endregion
//#region .NET 4.0
//// For field access it's 5x faster than the 3.5 method and 1.2x than "simple" method. For property access nearly same speed (1.1x faster).
//expr = Expression.Assign(propertyOrField, value);
//#endregion
return Expression.Lambda<Action<T, TValue>>(expr, parameter, value).Compile();
}
private static void GetMemberExpression<T, U>(Expression<Func<T, U>> expression, out ParameterExpression parameter, out Expression instance, out MemberExpression propertyOrField)
{
Expression current = expression.Body;
while (current.NodeType == ExpressionType.Convert || current.NodeType == ExpressionType.TypeAs)
{
current = (current as UnaryExpression).Operand;
}
if (current.NodeType != ExpressionType.MemberAccess)
{
throw new ArgumentException();
}
propertyOrField = current as MemberExpression;
current = propertyOrField.Expression;
instance = current;
while (current.NodeType != ExpressionType.Parameter)
{
if (current.NodeType == ExpressionType.Convert || current.NodeType == ExpressionType.TypeAs)
{
current = (current as UnaryExpression).Operand;
}
else if (current.NodeType == ExpressionType.MemberAccess)
{
current = (current as MemberExpression).Expression;
}
else
{
throw new ArgumentException();
}
}
parameter = current as ParameterExpression;
}
#region .NET 3.5
// Based on https://stackoverflow.com/questions/321650/how-do-i-set-a-field-value-in-an-c-expression-tree/321686#321686
private static Action<T, TValue> FieldSetter<T, TValue>(FieldInfo field)
{
DynamicMethod m = new DynamicMethod("setter", typeof(void), new Type[] { typeof(T), typeof(TValue) }, typeof(FluentTools));
ILGenerator cg = m.GetILGenerator();
// arg0.<field> = arg1
cg.Emit(OpCodes.Ldarg_0);
cg.Emit(OpCodes.Ldarg_1);
cg.Emit(OpCodes.Stfld, field);
cg.Emit(OpCodes.Ret);
return (Action<T, TValue>)m.CreateDelegate(typeof(Action<T, TValue>));
}
// Based on https://stackoverflow.com/questions/208969/assignment-in-net-3-5-expression-trees/3972359#3972359
private static Expression FieldSetter(Expression left, Expression right)
{
return
Expression.Call(
null,
typeof(FluentTools)
.GetMethod("AssignTo", BindingFlags.NonPublic | BindingFlags.Static)
.MakeGenericMethod(left.Type),
left,
right);
}
private static void AssignTo<T>(ref T left, T right) // note the 'ref', which is
{ // important when assigning
left = right; // to value types!
}
#endregion
}