CODEWITHSUNDEEP
.netgenericsreflectionlambdaexpression-trees
shay__
shay__

Reputation: 3990

How to Return Expression<Func<TClass, TClassField>> when given only FieldInfo and TClass?

In runtime I have only TClass and a FieldInfo, and I need to generate a lambda expression that gets an instance of TClass and returns the correlate field. After constructing a MemberExpression I got stuck when trying to wrap the expression to Expression<Func<TClass, TClassField>>:

var res = Expression.Lambda<Func<TClass, TClassField>>(memberExpression, paramExp);
return res;

Because TClassField is not known during compile time. I need some strongly typed solution (no casting to object) due to 3rd party requirements. Is this even possible in C#?

EDIT I need something like this - 

private void User3rdPartyLibrary<TClass>(FieldInfo fi)
{
    //Goal: call _3rdParty.Method<TClass, TClassField>(expression)

    var memberExp = Expression.Field(Expression.Parameter(typeof(TClass)), fi);
    //var lambda = some magic that returns  Expression.Lambda<Func<TClass, TClassField>>
    //      where fi.FieldType == typeof(TClassField).

    //_3rdParty.Method(lambda);
}

Signature of the 3rd party method:

public void Method<TClass, TClassMember>(Expression<Func<TClass, TClassMember>> expression);

Upvotes: 0

Views: 54

Answers (1)

Moho
Moho

Reputation: 16553

public static void User3rdPartyLibrary<TClass>( FieldInfo fi )
{
    // check types; add descriptions to exceptions
    if( fi.ReflectedType != typeof( TClass ) )
    {
        throw new ArgumentException();
    }

    var pe = Expression.Parameter( typeof( TClass ) );
    var me = Expression.Field( pe, fi );
    var memberExpression = Expression.Lambda( me, pe );

    // GetMethod call inlined for illustrative purposes
    typeof( ThirdPartyClass ).GetMethod( "ThirdPartyMethod" )
        .MakeGenericMethod( typeof( TClass ), fi.FieldType )
        .Invoke( memberExpression );
}

Then call using reflection:

FieldInfo fi = <whatever>;

// again, GetMethod call inlined for illustrative purposes
typeof( YourType ).GetMethod( "User3rdPartyLibrary" )
    .MakeGenericMethod( fi.ReflectedType )
    .Invoke( fi );

Upvotes: 2

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