[英]Handle null values of nested objects in C# expression tree
我已經搜索過,並且發現了與我的問題有關的類似帖子,但是似乎沒有什么可以解決我的問題。
我對C#相當陌生,這是我首次嘗試構建表達式樹。 (請放輕松;-)
我正在嘗試創建一個表達式樹,該樹一旦編譯,就可以過濾一組數據上的值。
這是我的表達方法:
private static Expression<Func<TItem, bool>> CreateFilterExpression<TItem>(string propertyName, string expressionType, dynamic filterValue)
{
if (param == null)
{
param = Expression.Parameter(typeof(TItem), "item");
}
MemberExpression member = GetMemberExpression<TItem>(propertyName);
//When we call our method, we need to evaluate on the same type
//we convert the filter value to the type of the property we are evaluating on
dynamic convertedValue = Convert.ChangeType(filterValue, member.Type);
MethodInfo method = member.Type.GetMethod(expressionType, new[] { member.Type });
ConstantExpression constantValue = Expression.Constant(convertedValue, member.Type);
Expression containsMethodExp;
if (expressionType == "NotEqual")
{
method = member.Type.GetMethod("Equals", new[] { member.Type });
}
if (member.Type.ToString().ToLower() == "system.string")
{
//We need to compare the lower case of property and value
MethodCallExpression propertyValueToLowerCase = Expression.Call(member, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
MethodCallExpression filterValueToLowerCase = Expression.Call(constantValue, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
containsMethodExp = Expression.Call(propertyValueToLowerCase, method, filterValueToLowerCase);
}
else if (member.Type.ToString().ToLower() == "system.datetime")
{
//we need to compare only the dates
MemberExpression dateOnlyProperty = Expression.Property(member, "Date");
containsMethodExp = Expression.Call(dateOnlyProperty, method, constantValue);
}
else
{
containsMethodExp = Expression.Call(member, method, constantValue);
}
if (expressionType == "NotEqual")
{
containsMethodExp = Expression.Not(containsMethodExp);
}
return Expression.Lambda<Func<TItem, bool>>(containsMethodExp, param);
}
private static MemberExpression GetMemberExpression<TItem>(string propertyName)
{
if (param == null)
{
param = Expression.Parameter(typeof(TItem), "item");
}
MemberExpression member = null;
//Check if we have a nested property
if (propertyName.Contains('.'))
{
Expression nestedProperty = param;
string[] properies = propertyName.Split('.');
int zeroIndex = properies.Count() - 1;
for (int i = 0; i <= zeroIndex; i++)
{
if (i < zeroIndex)
{
nestedProperty = Expression.PropertyOrField(nestedProperty, properies[i]);
}
else
{
member = Expression.Property(nestedProperty, properies[i]);
}
}
}
else
{
member = Expression.Property(param, propertyName);
}
return member;
}
用法示例如下所示:
var lambda = CreateFilterExpression<T>("Some.Nested.Object", "Equals", "Some value");
var compiled = lambda.Compile();
gridData = gridData.Where(compiled);
我試圖最終綁定到網格的數據示例如下所示:
public class Some : BaseClass
{
public decimal NumberAvailable { get; set; }
public DateTime EffectiveDate { get; set; }
public Batch Batch { get; set; }
public decimal Price { get; set; }
public decimal Limit { get; set; }
public NestedClass Nested { get; set; }
public int? CompanyId { get; set; }
public decimal Amount { get; set; }
}
public class NestedClass : BaseClass
{
public int RequestId { get; set; }
public string Code { get; set; }
public string Company { get; set; }
public string Reference { get; set; }
}
當我們在某個對象(例如“ Some.Nested = null”)上具有空值,然后嘗試將“引用”轉換為小寫時,就會出現問題。 這里:
MethodCallExpression propertyValueToLowerCase = Expression.Call(member, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
這是調試器中的結果:
如何檢查嵌套對象上的null值,如果它為null,則返回空字符串?
我希望我能很好地解釋我的問題。 先感謝您!
您想要做的是生成一個這樣的表達式:
Some == null ? null : Some.Nested == null ? null : Some.Nested.Object
不幸的是,它不再是成員表達式,因此GetMemberExpression
對此不起作用。 相反,您需要一連串條件表達式,一次可以訪問一個以上的級別。
一旦有了,就可以執行<memberExpression> ?? string.Empty
<memberExpression> ?? string.Empty
獲取可以安全操作的字符串。
要生成后一個表達式,可以使用Expression.Coalesce
:
Expression.Coalesce(memberExpression, Expression.Constant(string.Empty))
對於成員表達式本身,您可以編寫如下內容:
Expression AccessMember(Expression obj, string propertyName)
{
string[] parts = propertyName.Split(new char[] { '.' }, 2);
Expression member = Expression.PropertyOrField(obj, parts[0]);
if (parts.Length > 1)
member = AccessMember(member, parts[1]);
return Expression.Condition(Expression.Equal(obj, Expression.Constant(null)),
Expression.Constant(null, member.Type), member);
}
可以這樣使用:
string path = "Some.Nested.Object";
string[] parts = path.Split(new char[] { '.' }, 2);
ParameterExpression param = Expression.Parameter(typeof(T), parts[0]);
Expression memberAccess = AccessMember(param, parts[1]);
然后, memberAccess
將恰好是上面鏈接的條件表達式。
合並到您的函數中(現在僅簡化為字符串),看起來可能像這樣:
Expression<Func<TObj, bool>> BuildFilterExpression<TObj, TMember>(string propertyPath, TMember comparisonValue, TMember defaultValue)
{
string[] parts = propertyPath.Split(new char[] { '.' }, 2);
ParameterExpression param = Expression.Parameter(typeof(TObj), parts[0]);
// get member access expression
Expression memberExpression = AccessMember(param, parts[1]);
// coalesce the member with the default value
memberExpression = Expression.Coalesce(memberExpression, Expression.Constant(defaultValue));
// get the comparison value as expression
Expression comparisonExpression = Expression.Constant(comparisonValue);
// type specific logic
if (memberExpression.Type == typeof(string))
{
MethodInfo toLowerMethod = typeof(string).GetMethod("ToLower", Type.EmptyTypes);
memberExpression = Expression.Call(memberExpression, toLowerMethod);
comparisonExpression = Expression.Call(comparisonExpression, toLowerMethod);
}
// create the comparison expression
Expression filterExpression = Expression.Equal(memberExpression, comparisonExpression);
return Expression.Lambda<Func<TObj, bool>>(filterExpression, param);
}
像這樣使用:
BuildFilterExpression<SomeType, string>("Some.Nested.Object", "foo bar", string.Empty)
…本質上會創建以下lambda表達式:
(Some) => ((Some == null ? null : Some.Nested == null ? null : Some.Nested.Object) ?? string.Empty).ToLower() == "foo bar"
上面的代碼假定對於屬性表達式Some.Nested.Object
, Some
是要傳遞給lambda的對象,因此要訪問的第一個屬性是Nested
。 原因是我根本不了解您的示例對象結構,因此我不得不提出一些建議。
如果希望Some
是傳遞的對象訪問的第一個屬性,則可以輕松更改它。 為此,請修改BuildFilterExpression
的開頭,以便不拆分propertyPath
。 將一些隨機名稱(甚至沒有名稱)傳遞給Expression.Parameter
,並將完整的propertyPath
傳遞給AccessMember
:
// don’t split up the propertyPath
// let’s call the parameter `obj`
ParameterExpression param = Expression.Parameter(typeof(TObj), "obj");
// get member access expression—for the full property path
Expression memberExpression = AccessMember(param, propertyPath);
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.