`Type.GetCustomAttributes()` can be slow when not passing an `attributeType` parameter.

[eiriktsarpalis]

Consider the following benchmark:

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Reflection;
using System.Runtime.CompilerServices;

BenchmarkRunner.Run<MyBench>(args: args);

public class MyBench
{
    private readonly Type _type = typeof(ValueType);
    private readonly Type _requiredMemberAttribute = typeof(RequiredMemberAttribute);

    [Benchmark(Baseline = true)]
    public RequiredMemberAttribute? GetCustomAttributeTypeArgument()
    {
        return (RequiredMemberAttribute?)_type.GetCustomAttributes(_requiredMemberAttribute, inherit: false).FirstOrDefault();
    }

    [Benchmark]
    public RequiredMemberAttribute? GetCustomAttributeNoTypeArgument()
    {
        foreach (object attribute in _type.GetCustomAttributes(inherit: false))
        {
            if (attribute.GetType() == _requiredMemberAttribute)
            {
                return (RequiredMemberAttribute)attribute;
            }
        }

        return null;
    }

    [Benchmark]
    public CustomAttributeData? GetCustomAttributesData()
    {
        foreach (CustomAttributeData attrData in _type.GetCustomAttributesData())
        {
            if (attrData.AttributeType == _requiredMemberAttribute)
            {
                return attrData;
            }
        }

        return null;
    }
}

Method	Mean	Error	StdDev	Ratio	RatioSD
GetCustomAttributeTypeArgument	388.8 ns	2.48 ns	2.32 ns	1.00	0.00
GetCustomAttributeNoTypeArgument	1,932.7 ns	10.42 ns	9.24 ns	4.97	0.04
GetCustomAttributesData	3,759.0 ns	26.10 ns	21.80 ns	9.67	0.09

Discovered while investigating #89794

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Issue Details

---

Consider the following benchmark:

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;
using System.Reflection;
using System.Runtime.CompilerServices;

BenchmarkRunner.Run<MyBench>(args: args);

public class MyBench
{
    private readonly Type _type = typeof(ValueType);
    private readonly Type _requiredMemberAttribute = typeof(RequiredMemberAttribute);

    [Benchmark(Baseline = true)]
    public RequiredMemberAttribute? GetCustomAttributeTypeArgument()
    {
        return (RequiredMemberAttribute?)_type.GetCustomAttributes(_requiredMemberAttribute, inherit: false).FirstOrDefault();
    }

    [Benchmark]
    public RequiredMemberAttribute? GetCustomAttributeNoTypeArgument()
    {
        foreach (object attribute in _type.GetCustomAttributes(inherit: false))
        {
            if (attribute.GetType() == _requiredMemberAttribute)
            {
                return (RequiredMemberAttribute)attribute;
            }
        }

        return null;
    }

    [Benchmark]
    public CustomAttributeData? GetCustomAttributesData()
    {
        foreach (CustomAttributeData attrData in _type.GetCustomAttributesData())
        {
            if (attrData.AttributeType == _requiredMemberAttribute)
            {
                return attrData;
            }
        }

        return null;
    }
}

Method	Mean	Error	StdDev	Ratio	RatioSD
GetCustomAttributeTypeArgument	388.8 ns	2.48 ns	2.32 ns	1.00	0.00
GetCustomAttributeNoTypeArgument	1,932.7 ns	10.42 ns	9.24 ns	4.97	0.04
GetCustomAttributesData	3,759.0 ns	26.10 ns	21.80 ns	9.67	0.09

Discovered while investigating #89794

Author:	eiriktsarpalis
Assignees:	-
Labels:	area-System.Reflection, untriaged, needs-area-label
Milestone:	-

[ericstj]

I'm not surprised. There's more work to do and more data to return when not filtering to specific type. Moving out to 9.0.0 as this doesn't appear to be a regression.